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. 2025 Mar 25;28(4):112290. doi: 10.1016/j.isci.2025.112290

Peripheral biological correlates of suicidality in children and adolescents: A systematic review and meta-analysis

Thomas K Pak 1,4, Emine Rabia Ayvaci 1,2,4, Thomas Carmody 1,3, Limi Jamma 1, Zihang Feng 1, Arya Nekovei 1, Graham Emslie 1,2, Madhukar H Trivedi 1,5,
PMCID: PMC12013498  PMID: 40264798

Summary

A systematic review and meta-analysis were conducted to identify peripheral biological correlates of suicidality in children and adolescents. The review was pre-registered through PROSPERO (CRD42023417128) and included four databases (PubMed, Cochrane Library, Embase, and PsycINFO). From 27,977 non-duplicated articles, 102 full-text studies were selected. Studies investigated suicide attempts (n = 52), suicidal ideation (n = 42), or individuals with suicidal ideation or attempts grouped together (n = 22). Seropositive toxoplasmosis, cortisol, neutrophil, and neutrophil to lymphocyte ratio (NLR) exhibited significant effect size after Bonferroni correction. Effect sizes for biological correlates of suicidality were pooled using Cohen’s d (effect size = −0.04, 95% confidence interval [CI]: −1.36 to 1.27) and odds ratio (effect size = −0.31, 95% CI: −1.06 to 0.42). Meta-regression analysis revealed that type of suicidality, type of control, means collected, and sample size significantly impacted the pooled effect size. Analysis showed significant publication bias and heterogeneity, as well as notable moderators and potential biomarkers for future research.

Subject areas: Public health, Psychology

Graphical abstract

graphic file with name fx1.jpg

Highlights

  • We sought to determine peripheral biological correlates of suicidality in children

  • Cortisol and neutrophil were significantly associated with suicidality

  • The type of suicidality and type of control affected the strength of the association

  • The studies had significant publication bias and heterogeneity


Public health; Psychology

Introduction

Suicide is the third leading cause of death among children and adolescents in the United States.1 The impact of youth suicide on public health, healthcare costs, and society is substantial.2 Early identification and prevention of suicidal behaviors in youth are critical, offering the potential to save lives through timely intervention. Identifying suicide risk remains a challenge in clinical practice, predominantly relying on clinical interviews and associated clinical risk factors. Integrating biomarkers into this process has the potential to enhance risk prediction and subsequently prevent suicidal behaviors and attempts.3 For the development of clinically useful biomarkers, identifying valid and reliable biological correlates is essential.4

Despite an increased number of research studies on suicidal ideation and behavior (suicidality) in adults,5,6 there is a noticeable gap in research on biological correlates of suicidality in children and adolescents.7 Research in children and adolescents is critical as there are developmental differences in children and adolescents that can affect the expression and detection of biomarkers.8,9 Thus, this manuscript aims to expand the current research on suicidality in children and adolescents by identifying biological correlates associated with suicidality, and moderators for biological correlates associated with suicidality.

Previous reviews on suicidality in youth focused on clinical correlates,7 peripheral correlates,10,11 and neural correlates.10,12 A scoping review by Sparrow-Downes et al. provided information on peripheral and neural correlates of self-harm in children and adolescents ages 3 to 19 years and identified both limited replication of studies and a predominance of female sex in study samples in the literature.10 Two other systematic reviews in youth looked at biological correlates of suicidality, but each review only noted two studies that examined biological correlates of suicidality.11,13 Other non-systematic reviews focused on genetic correlates of suicidality in children and adolescents.14,15,16 Adult literature is more robust, with systematic reviews and meta-analyses of suicidality and a variety of biological correlates, including genes,17 Brain-derived neurotrophic factor (BDNF),18,19 Electroencephalography (EEG),20 cortisol levels,21 leptin,22 cerebrospinal fluid (CSF) monoamines,23 lipid,24 and inflammation.25,26,27 Some of these meta-analyses included patients younger than 18 years but did not conduct subgroup analyses for children and adolescents. Given the growing body of research on suicidality in this age group,7 there is a critical need for a systematic review examining biological correlates in these populations.

To address this gap in the literature, the authors conducted a systematic review of suicidality in children and adolescents and a meta-analysis of biological correlates associated with suicidality. This study is structured around three key objectives: First, descriptive characteristics of the literature focusing on suicidality in children and adolescents were reviewed, considering demographics, sample size, study design, type of suicidality, and type of biological correlates. Next, a meta-analysis for each biological correlate with results from at least three studies was conducted. Finally, a meta-regression was conducted to identify study characteristics that are moderators of effect size.

Results

Systematic review

The systematic review yielded 27,977 non-duplicated articles, of which 591 were included based on titles and abstracts. After a full-text review, 102 articles were included for systematic review.28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128 The PRISMA flow diagram depicts the search and selection process (Figure 1). Sixty-nine studies had data for meta-analysis, and 19 biological correlates had enough studies for individual meta-analysis.129

Figure 1.

Figure 1

PRISMA flow diagram for systematic review

Table 1 presents the descriptive characteristics of each study, while Table 2 shows the summary characteristics of the studies. The extracted research articles are from 1985 to 2023, with a large number published in the last nine years (n = 56, 50%).

Table 1.

Characteristics and quality of individual studies

First Author Year PMID or DOI Study Numbera Race and Ethnicity Sample Size (N) Age Range Mean Ageb Sex % Female Study Design Specific Biological Correlate JBI (%)
1. Cytokines

Jha 2020 31756521 39 Black, White, and other 109 10-25 years 17.2 Both Sex 59 Case-control Bio-Plex Pro Human Chemokine 40-plex assay: (chemokine, interleukin, MCP, TNF-a, Eotaxin, interferon) 80.0
Amitai 2020 31887416 48 Not reported 92 6-18 years 13.9 Both Sex 62 Prospective cohort TNF-a, IL-6, IL-1β levels 81.8
Amitai 2020 32747326 6 Not reported 92 Not reported 13.9 Both Sex 62 Prospective cohort IL-6, IL-1β, and TNF-α level 63.6
Clayton 2023 36853582 53 White, Black, Hispanic, and other 157 12-16 years 14.7 Female 100 Prospective cohort Saliva IL-1β, IL-6, and TNF-α 81.8
Gabbay 2009 19702494 4 Not reported 45 12-19 years 16 Both Sex 60 Case-control IFN-γ, TNF-α, IL-6, IL-1β, IL-4, IFN-γ 80.0

2. Endocrine System

Dahl 1992 1644723 75 Black, White, and other 44 12-18 years 14.8 Both Sex 52 Case-control Growth hormone 80.0
Garcia 1991 1905294 98 Black, White, and other 55 6-12 years 10.1 Both Sex 38 Case-control Thyroid-stimulating hormone response to Thyroid releasing hormone (plasma) 80.0
Ryan 1988 2975304 43 Black, White, and other 48 12-17 years 15.3 Both Sex 41 Case-control Mean GH response to desmethylimipramine 80.0
Sokolov 1994 8005899 18 Not reported 40 14-20 years 16.9 Both Sex 42 Case-control Basal serum thyrotropin, T4, fT4, triiodothyronine (T3), reverse-T3, free thyroxine index (FTI), and T3 resin uptake 100.0
Lebowitz 2020 30699863 58 Not reported 168 7-16 years 11 Both Sex 51 Cross-sectional Saliva oxytocin levels 87.5
Martin 1997 9359983 73 Not reported 160 15-19 years 16 Both Sex 50 Cross-sectional Progesterone levels 87.5
Gokalp 2020 3965035 54 MENA and other 179 12-19 years 15.8 Female 100 Case-control Serum thyroid-stimulating hormone and free T3 and free T4 80.0

3. Genes

Cicchetti 2010 19779024 51 White, Black, Hispanic, and other 850 6-13 years 9.2 Both Sex 46 Case-control Serotonin transporter gene-linked promoter region (5-HTTLPR) polymorphism 100.0
Ran 2020 31629822 74 Asian 224 13-25 years 19 Both Sex 61 Case-control Serotonin transporter gene SLC6A4 80.0
Zalsman 2004 15565493 38 MENA 236 15-24 years Not reported Both Sex Not reported Case-control DRD4 receptor gene exon III polymorphism: greater/equal or lower than 7 repeats of DRD4 90.0
Kronenberg 2007 18315446 81 MENA 74 7-19 years Not reported Both Sex 59 Case-control 5-HTTLPR serotonin transporter polymorphism 80.0
Brezo 2009 19381154 34 White 1255 Not reported Not reported Both Sex Not reported Prospective cohort Tag serotonergic SNPs 81.8
Brent 2010 20008943 11 White 155 12-18 years 15.7 Both Sex 70 Retrospective cohort FKBP5 rs1360780TT and rs3800373GG genotypes 72.7
Zalsman 2011 20873971 13 White 211 13-22 years 15.9 Both Sex 58 Case-control Polymorphisms of the serotonergic pathways (HTR2A, 5-HTTLPR, and MAOA) 60.0
Fiori 2020 21152090 12 White 1255 6 years Not reported Both Sex 59 Prospective cohort Genotyped 63 polymorphisms 72.7
Doorley 2016 27267123 87 White and other 76 13-18 years 14.9 Both Sex 71 Case-control Dopamine D4 receptor polymorphism. carrier of DRD4 or not 90.0
Blázquez 2016 27309038 60 White 46 10-17 years 15 Both Sex 78 Prospective cohort Fluoxetine transportation (ABCB1) gene polymorphism. Dichotomous 63.6
Mirkovic 2017 28902619 3 Not reported 248 13-27 years 15.4 Both Sex 80 Case-control 22 SNPs in 12 genes 80.0
Sarmiento-Hernández 2019 30724325 8 Hispanic 435 11-18 years 14.3 Both Sex 64 Case-control 5-HTTLPR: genotype frequency 90.0
Brick 2019 30769295 61 White 3564 8-21 years 13.7 Both Sex 50 Cross-sectional Manhattan plot of GWAS, SNPs 87.5
Koyama 2020 31870620 72 Not reported 158 9-17 years 13.9 Both Sex 50 Case-control Impulsive aggression gene panel (CRH, CRHR2, MC2R, OXTR, BDNF) 90.0
Acikel 2020 31916884 92 Not reported 203 12-18 years 15.2 Both Sex 75 Case-control Leptin receptor polymorphism - rs1171276. homo/het vs. control 90.0
Hill 2020 32745832 89 Not reported 330 8-18 years 11.57 Both Sex 49 Prospective cohort Genetic variation (ANKK1, DRD2, COMT, SLC6A4, HTR2C) 72.7
Russel 2021 33892675 102 Not reported 8237 16-24 years Not reported Both Sex Not reported Prospective cohort SNP-based heritability and Polygenic Risk Score 72.7
Sanabrais-Jiménez 2022 34342556 16 Hispanic 197 12-17 years 15 Both Sex 64 Case-control SLC6A4, DRD2, COMT and MAOA genes 90.0
Lee 2022 35216811 42 White, Black, Asian, Hispanic, and other 11878 9-10 years 9.91 Both Sex 47 Prospective cohort Polygenic risk score for psychiatric conditions 72.7
Li 2013 24229544 35 White 659 Not reported 15.5 Both Sex 44 Prospective cohort Polymorphism in the serotonin transporter linked promoter region gene (5-HTTLPR) 63.6
Laas 2014 24331455 52 White 1176 9-25 years 18 Both Sex 55 Prospective cohort Neuropeptide S receptor gene polymorphism 81.8
Lvovs 2022 34924075 25 White 10000 15 years 15 Both Sex Not reported Prospective cohort Cholecystokinin B receptor gene polymorphism 63.6
Haeffel 2008 18181793 9 White 176 Not reported 16.2 Male 0 Cross-sectional Polymorphisms in the dopamine transporter gene 87.5

4. Gene expression

Kong 2021 33971247 19 Not reported 200 11-18 years 15 Both Sex 87 Case-control BDNF gene expression 80.0
El Gayed 2021 34568691 56 MENA 160 12-24 years 17.6 Both Sex 51 Case-control CSMD1 gene’s mRNA and protein expression 80.0
Perret 2023 36712964 17 Not reported 149 Not reported 10.47 Both Sex Not reported Prospective cohort Epigenome-wide DNA methylation 81.8

5. Immune System

Ambrosini 1992 1314256 69 Not reported 32 Not reported 13.9 Both Sex 56 Case-control Platelet imipramine binding 70.0
Carstens 1988 2834765 20 Not reported 48 Not reported 15.4 Both Sex 62 Case-control Equilibrium dissociation constant (Kd), and total number of binding sites (Bmax): 3H-p-aminoclonidine binding to platelets. 3H-p-aminoclonidine, 3H-imipramine binding, 3H-DHA binding to lymphocyte membranes. 100.0
Pine 1995 7755125 80 Not reported 121 12-18 years 15.7 Both Sex 85 Case-control Density of platelet [3H] imipramine binding sites 80.0
Soreni 1999 10459397 31 Not reported 19 13-20 years 16.6 Both Sex 47 Case-control [3H] PK 11195 binding to platelet membrane 100.0
Ragolsky 2013 23410141 90 Not reported 821 12.5–18 years 15.3 Both Sex 51 Case-control Platelet counts 90.0
Ucuz 2020 32650198 24 Not reported 302 11-18 years 15.6 Both Sex 78 Case-control Hemogram parameters - white blood cell, red blood cell, hemoglobin, neutrophil lymphocyte ratio 90.0
Amitai 2022 35470013 59 Not reported 160 6-18 years 13.9 Both Sex 62 Prospective cohort Neutrophil/lymphocyte ratio (NLR), Platelet/lymphocyte ratio (PLR) 72.7
Önen 2021 38765642 22 Not reported 148 9-16 years 14 Both Sex 70 Case-control Neutrophil to lymphocyte ratio; mean platelet volume; platelet to lymphocyte ratio; hemoglobin; hematocrit; white blood cell; red cell distribution width 80.0

6. Infection Serology

Coryell 2016 27045220 100 Not reported 110 15-20 years 18.7 Both Sex 71 Case-control Serum toxoplasmosis titers or presence 72.7
Sapmaz 2019 31238296 76 Not reported 73 11-18 years 15 Both Sex 75 Case-control Seropositivity for Toxoplasma gondii 90.0
Sari 2019 31688493 14 Not reported 100 12-18 years Not reported Both Sex 86 Case-control Toxoplasma gondii IgM and IgG antibodies 90.0
Yucel 2020 33378287 7 Not reported 53 12-18 years 15.9 Both Sex 63 Case-control Toxoplasma gondii serology. (specific with IgG) 90.0
Bayturan 2022 35821494 77 Not reported 76 11-18 years 15.1 Both Sex 83 Case-control HSV1, CMV, EBV, HHV6 seropositivity and serum antibodies 90.0

7. Inflammatory Marker

Falcone 2010 20559426 85 Not reported 84 12-18 years 14.3 Both Sex 42 Case-control Serum S100B levels 90.0
Falcone 2015 25669696 79 White, Hispanic, Asian, and Black 115 7-18 years 15 Both Sex 46 Case-control Serum S100B levels 90.0
Liu 2020 33381770 27 Black, White, and other 64 12-20 years 15.1 Both Sex 68 Cross-sectional C-reactive protein 100.0
Liu 2021 34166062 55 Black, White, and other 127 12-20 years 15.2 Both Sex 61 Case-control C-reactive protein 100.0

8. Metabolism

Glueck 1994 8065845 45 Black, White, and other 1268 5-18 years 12.3 Both Sex 38 Case-control Total cholesterol and triglyceride levels 90.0
Plana 2010 20047063 99 Not reported 120 8-18 years 15.3 Both Sex 73 Case-control Total serum cholesterol 90.0
Gokalp 2020 32384132 93 Not reported 415 7-18 years 15 Both Sex 95 Case-control Serum vitamin D, calcium, and phosphorus levels 90.0
Wu 2023 36493942 21 Not reported 90 13-18 years 15 Both Sex 68 Case-control Intestinal permeability (plasma levels of zonulin, I-FABP, LPS and claudin-5) 80.0
Apter 1999 10459404 83 MENA and other 152 12-21 years 16.1 Both Sex 52 Case-control Serum cholesterol levels 80.0
Kong 2022 35899094 10 Not reported 533 13-25 years 17.6 Both Sex 73 Case-control Uric acid - serum 90.0

9. Neuromodulators

Gulec 2010 DOIc 66 Not reported 55 15-18 years 16.5 Both Sex 37 Case-control Neuropeptide Y (NPY) levels - plasma 80.0
Venne 2021 32961416 63 Not reported 129 12-17 years 14.9 Female 100 Case-control Plasma beta endorphin levels 100.0

10. Neurotrophin

Kavurma 2017 29017139 37 Not reported 105 12-18 years 15.1 Both Sex 72 Case-control Serum BDNF levels 90.0
Bilgiç 2020 32027188 41 Not reported 110 11-19 years 15.6 Both Sex 76 Case-control Serum BDNF, GDNF, NGF, and NTF3 levels 90.0
Lee 2020 32090756 40 Not reported 135 12-17 years 14.8 Both Sex 62 Prospective cohort BDNF levels 72.7

11. Serotonin System

Sallee 1998 9666634 70 Black, White, and other 46 9-17 years 14.2 Both Sex 47 Case-control Platelet 5-HTPR kinetic analysis in reduced binding capacity 90.0
Pfeffer 1998 9787881 64 White, Black, and Hispanic 110 6-12 years 8.93 Both Sex 34 Case-control Whole blood tryptophan 90.0
Clark 2003 12914891 86 White and Black 54 Not reported 16.2 Both Sex 60 Prospective cohort Tryptophan ratio to other amino acids in the serum (index of serotonin precursor available to the brain), along with serum tryptophan 63.6
Tyano 2006 16076550 67 MENA and other 211 Not reported 15.9 Both Sex 58 Case-control Plasma serotonin levels 90.0
Bradley 2015 25865484 97 Black, White, and other 30 12-18 years 15.8 Both Sex 67 Case-control Tryptophan, kynurenine, 3-hydroxyanthranilic acid (plasma) 90.0
Zhou 2006 16828946 68 Not reported 100 15-19 years 17.2 Male 0 Case-control Plasma serotonin levels 90.0
Modai 1989 2812295 82 Not reported 34 Not reported Not reported Not reported Not reported Case-control Serotonin uptake of platelets 80.0

12. Stress Response System

Dahl 1992 1420629 95 Black, White, and other 61 11-17 years 15 Both Sex 61 Case-control Cortisol levels (serum), dexamethasone suppression test 100.0
Birmaher 1992 1636803 94 Black, White, and other 82 11-18 years 15.1 Both Sex 42 Case-control Dexamethasone-induced cortisol levels 80.0
Dahl 1991 1892959 1 Black, White, and other 59 12-18 years 15.03 Both Sex 61 Case-control Cortisol levels 80.0
Pfeffer 1991 2049489 88 White, Black, Hispanic, and other 49 6-12 years 10.6 Both Sex 29 Case-control Cortisol levels (plasma) and dexamethasone suppression test 90.0
Weller 1990 2136395 33 White 18 7-18 years 13 Both Sex 22 Cross-sectional Dexamethasone suppression test 70.0
Puig-Antich 1989 2673131 30 White, Black, and Hispanic 45 6-12 years 9.2 Both Sex 39 Cross-sectional Plasma cortisol 70.0
Dahl 1989 2763857 29 Not reported 88 12-18 years 15.2 Both Sex 45 Case-control Cortisol secretion (plasma) 90.0
Rao 1996 8879467 96 White and other 63 12-18 years 63 Both Sex 61 Prospective cohort Plasma cortisol levels 81.8
Mathew 2003 12784120 36 Not reported 77 Not reported 25.52 Both Sex Not reported Prospective cohort 24-h cortisol secretion 72.7
Klimes-Dougan 2019 30590339 46 White, Black, Asian, and other 162 12-19 years 16.4 Both Sex 67 Case-control Salivary cortisol levels in context of Trier social stress test 90.0
Shalev 2019 31299399 28 White and other 223 6-25 years 12.3 Both Sex Not reported Case-control Cortisol response 100.0
Denton 2021 33876491 26 Black and other 50 8-17 years 14 Both Sex 66 Cross-sectional Cortisol levels (saliva) 87.5
Robbins 1985 DOId 91 Not reported 45 13-18 years Not reported Both Sex Not reported Case-control Dexamethasone suppression test 90.0
Young 2010 20419739 101 Not reported 501 Not reported 15.3 Both Sex 42 Cross-sectional Morning cortisol levels (saliva) 87.5
Ghaziuddin 2014 24524706 32 White and other 44 13-17 years 15.5 Both Sex 66 Case-control Cortisol response 100.0
Giletta 2014 24958308 57 White, Black, Asian, Hispanic, and other 138 12-16 years 14.3 Female 100 Prospective cohort Salivary cortisol levels to Trier social stress test 63.6
Beauchaine 2015 25208812 71 Not reported 57 13-17 years 16 Female 100 Case-control Dexamethasone suppression test 90.0
Eisenlohr-Moul 2018 30267013 44 White, Black, Asian, Hispanic, and other 220 12-16 years 14.6 Female 100 Prospective cohort Salivary cortisol to Trier social stress test 63.6
Bendezú 2021 33762041 5 White, Black, Asian, Hispanic, and other 241 12-17 years 14.7 Female 100 Prospective cohort Salivary cortisol 72.7

13. Multiple Biological Categories

Melhem 2018 28135675 23 White and other 115 15-30 years 23 Both Sex 43 Case-control Stress Response System: hair cortisol concentration, cellular measures of glucocorticoid receptor sensitivity Inflammatory Markers: plasma C-reactive protein. Cytokines: stimulated production of IL-6 90.0
Zakowicz 2023 37041682 65 Not reported 79 Not reported 15 Both Sex 55 Case-control Neurotrophins: BDNF, proBDNF, p75NTR Inflammatory Marker: S100B plasma levels 90.0
Zhang 2022 36733417 15 Asian 179 13-18 years 15.4 Both Sex 71 Cross-sectional Endocrine: thyroid-stimulating hormone Metabolism: lipids 87.5
Kruesi 1992 1376104 2 Black, White, and other 29 6-17 years 11.3 Both Sex 7 Prospective cohort Serotonin system: cerebrospinal fluid 5-hydroxyindoleacetic acid, Metabolism: homovanillic acid 72.7
Karadeniz 2020 32496844 84 Not reported 50 12-18 years 14.7 Both Sex 67 Case-control Neuromodulators: serum nesfatin-1, ghrelin, Metabolism: lipid levels 80.0
Bilginer 2021 34185736 62 Not reported 74 12-17 years 14.9 Both Sex 83 Case-control Inflammatory marker: Serum S100B

Stress response system: malondialdehyde (MDA), total oxidant status (TOS), and total antioxidant status (TAS)
80.0
Chin Fatt 2022 36120101 47 Not reported 14 12-18 years 14.4 Female 100 Case-control Immune system and inflammatory markers: panel of 30 antibodies. Includes immune dysregulation markers (e.g., CCR2, CXCR5, chemokine receptors) and inflammatory markers (CD294) 90.0
Zalsman 2005 15657646 78 MENA 60 Not reported 17 Both Sex 50 Case-control Genes: serotonin transporter promoter polymorphism (5-HTTLPR), Immune System: platelet serotonin transporter (SERT) binding, number of platelets 90.0
Si 2020 32559305 49 Asian 682 Not reported 16.9 Both Sex 56 Cross-sectional Genes: TNF-RII gene variations on lipid levels Metabolism: Lipids 75.0
Goldstein 2016 26646032 50 White and other 123 13-28 years 20.4 Both Sex 38 Prospective cohort Cytokines: serum levels of IL-6, TNF-α, Inflammatory marker: high-sensitivity C-reactive protein (hsCRP) 72.7

5-HTPR, Serotonin transporter; 5-HTTLPR, Serotonin transporter linked promoter region; ANKK1, Ankyrin repeat and kinase domain containing 1; BDNF, Brain-derived neurotrophic factor; CMV, Cytomegalovirus; COMT, Catechol-O-methyltransferase; CRH, Corticotropin-releasing hormone; CRHR2, CRH receptor 2; CSMD1, CUB and Sushi multiple domains 1; DNA, Deoxyribonucleic acid; DOI, Digital object identifier; DRD4, Dopamine Receptor D4 Gene; EBV, Epstein Barr virus; FKBP5, FK506-Binding Protein 5; fT4, Free Thyroxine; GDNF, Glial cell line-derived neurotrophic factor; GH, Growth Hormone; GWAS, Genome-Wide association study; HHV6, Human herpesvirus 6; HSV1, Herpes Simplex Virus 1; HTR2A, 5-hydroxytryptamine receptor 2A; HTR2C, 5-hydroxytryptamine receptor 2C; I-FABP, Intestinal fatty acid binding protein; IFN, Interferon; IgG, Immunoglobulin G; IgM, Immunoglobulin M; IL, Interleukin; JBI, Joanna Briggs Institute critical appraisal tool; LPS, Lipopolysaccharide; MAOA, Monoamine oxidase A; MC2R, Adrenocorticotropic hormone receptor; MCP, Monocyte chemoattractant protein; MENA, Middle Eastern and North African descent; mRNA, messenger RNA; NGF, Nerve growth factor; NTF3, Neurotrophin-3; OXTR, Oxytocin receptor; p75NTR, p75 Neurotrophin receptor; SNP, Single nucleotide polymorphism; T4, Thyroxine; TNF, Tumor Necrosis Factor.

a

Each study was assigned a unique ID number for identification in Figure 2 and Supplementary Figures. These are not reference numbers.

b

Studies grouped by corresponding biological category. Some studies have examined multiple biological categories.

Table 2.

Summary characteristics of included studies

Study characteristics N %
Sex

Male only 2 2.0
Female only 8 7.8
Both sexes 91 89.2
Not specified 1 1.0

Age

Children (6–12 years) 5 4.9
Adolescents (13–18 years) 18 17.6
Children and adolescents (6–18 years) 53 52.0
Adolescents and young adults (13–30 years) 11 10.8
All three age groups (children, adolescents, young adults) 15 14.7

Race and ethnicitya

White 42 41.2
Black 26 25.5
Hispanic 12 11.8
Asian 9 8.8
Middle Eastern and North African 7 6.9
Other (race or ethnicity not listed above) 31 30.4
Not specified 47 46.1

Sample size

14-50 (small) 20 19.6
51-100 (medium) 25 24.5
101-250 (large) 39 38.2
251-1000+ (very large) 18 17.6

Sample sourcea

Community 67 65.7
Outpatient 49 48.0
Inpatient/Emergency Department 44 43.1
Not specified 1 1.0

Study design

Case-control 67 65.7
Prospective cohort 23 22.5
Cross-sectional 11 10.8
Retrospective cohort 1 1.0

Biological correlate categorya

Genes 25 24.5
Stress response system 21 20.6
Serotonin system 10 9.8
Immune system 10 9.8
Metabolism 10 9.8
Inflammatory markers 9 8.8
Endocrine system 8 7.8
Cytokines 7 6.9
Infection serology 5 4.9
Neuromodulators 3 2.9
Neurotrophin 4 3.9
Gene expression 3 2.9

Types of suicidalitya

Suicide attempt 52 51.0
Suicidal ideation 42 41.2
Suicide attempt and suicidal ideation grouped 22 21.6
a

Individual articles may have included multiple categories of these study characteristics.

Most studies included male and female subjects (n = 91, 89%). In addition, 53 articles (52%) included the age range of children (6–12 years old) and adolescents (13–18 years old). The race/ethnicity reported in most articles is White/Caucasian (n = 42, 41.2%), followed by Black/African American (n = 26, 25.5%). Forty-seven articles (46.1%) did not report race or ethnicity. The sample sizes varied widely, from 14 subjects to 11,878 subjects. Many of the larger studies came from national health registries. In terms of the research setting, 67 articles (65.7%) recruited patients from the community setting, 49 articles (48.0%) recruited patients from the outpatient clinics, and 44 articles (43.1%) recruited patients from the inpatient or Emergency Department. One of the articles did not specify the setting.

The majority of the extracted research articles were case controls (n = 67, 65.7%), followed by prospective cohort studies (n = 23, 22.5%), and then cross-sectional studies (n = 11, 10.8%). Regarding the biological correlate categories, the genes category was the largest of the twelve categories (n = 25, 24.5%), followed by the stress response system (n = 21, 20.6). For types of suicidality, the largest category was suicide attempt (n = 52, 51%), followed by suicidal ideation (n = 42, 41.2%), and grouped suicide attempt and suicidal ideation (n = 22, 21.6%).

Quality assessment of the studies

The quality assessment of the studies, using the Joanna Briggs Institute (JBI) Risk of Bias tool (Table 1), showed that most articles were of good quality (scored 70%–99% of the checklist), totaling 81 articles (79.4% of the research articles), and 10 articles (9.8% of research articles) were rated as excellent (scored 100% of the checklist). This reflects the general robustness of the studies extracted. Eleven articles were categorized as fair (scored 50%–70% of the checklist), and none of the studies fell into the poor category (scored 0%–50%).

Meta-analysis and meta-regression results

Sixty-nine studies, comprising 333 total effects, provided the data to run a meta-analysis and meta-regression using Cohen’s d or odds ratio. Nineteen individual biomarkers of suicidality met the criteria for individual meta-analysis (values from at least three individual studies129) and are shown in Table S1. Eight biological correlates were significantly associated with suicidality, displayed as forest and funnel plots (Figures 2 and 3), and 11 biological correlates were not significantly associated with suicidality, which are displayed as forest and funnel plots (Figures S1 and S2).

Figure 2.

Figure 2

Forest plot of effect size (Cohen’s d) of biological correlates of suicidality

(A) Forest plot for cortisol (p < 0.0001).

(B) Forest plot for dexamethasone suppression test (non-suppressors) (p = 0.013).

(C) Forest plot for neutrophil (p < 0.0001).

(D) Forest plot for platelet (p = 0.025).

(E) Forest plot for platelet to lymphocyte ratio (p = 0.046).

(F) Forest plot for neutrophil to lymphocyte ratio (p = 0.0023).

(G) Forest plot for seropositive toxoplasmosis (p < 0.0001).

(H) Forest plot for tryptophan (p = 0.045). The Black Diamond at the bottom is the pooled effect size. Each horizontal line represents a specific biomarker from a specific study. The study number and biomarker are listed on the left of the forest plot. The study numbers reflect the study numbers in Table 1 and are not reference numbers. The dots and whiskers at the center represent the effect size and 95% confidence interval. The right of the forest plot is the numerical data of the effect size and the 95% confidence interval. Cortisol, neutrophil, neutrophil to lymphocyte ratio, and seropositive toxoplasmosis remained significant after Bonferroni correction. Some study numbers are listed multiple times since different study characteristics were utilized for assessing biological correlates of suicidality (different types of suicidality and control). DST; dexamethasone suppression test. RE, random effect.

Figure 3.

Figure 3

Funnel plot of effect size (Cohen’s d) of biological correlates of suicidality

(A) Funnel plot for cortisol.

(B) Funnel plot for dexamethasone suppression test (non-suppressors).

(C) Funnel plot for neutrophil.

(D) Funnel plot for platelet.

(E) Funnel plot for platelet to lymphocyte ratio.

(F) Funnel plot for neutrophil to lymphocyte ratio.

(G) Funnel plot for seropositive toxoplasmosis.

(H) Funnel plot for tryptophan. Each dot represents an individual biological correlate from a study. The x axis is the effect Size (Cohen’s d) and the y axis is the standard error. DST, dexamethasone suppression test.

We identified four promising biological correlates for suicidality: cortisol (p < 0.0001), neutrophil (p < 0.0001), neutrophil to lymphocyte ratio (NLR) (p = 0.0023), and seropositive toxoplasmosis (p < 0.0001). These four correlates remained significant after adjustment for multiple comparisons with Bonferroni correction. Our sensitivity analysis showed that only neutrophil (p < 0.0001) and NLR (p = 0.0014) remained significant after accounting for the type of suicidality and accounting for studies that had multiple results.

We highlight the certainty of our results for individual biological correlates using the Grading of Recommendations, Assessment, Development, and Evaluations (GRADE) guideline (Table S1). While Egger’s test identified a few correlates with potential publication bias, this was skewed by the small sample size. Therefore, the analysis relied more heavily on visually inspecting the funnel plots (Figure 3 and Table S1). Of the biological correlates associated with suicidality, most had a small to moderate effect size of Cohen’s d, except for seropositive toxoplasmosis (effect size = 1.43, 95% confidence interval [CI]: 0.87 to 2.00, p < 0.0001). Cortisol levels had small confidence intervals and effect size of Cohen’s d with suicidality (effect size = 0.33, 95% CI: 0.21 to 0.45, p < 0.0001). Multiple cell blood count measurements showed significant effect sizes of Cohen’s d: neutrophil count (effect size = 0.47, 95% CI: 0.29 to 0.66, p < 0.0001), and NLR (effect size = 0.65, 95% CI: 0.23 to 1.07, p = 0.0023).

The overall pooled effect size (adjusted) for biological correlates and suicidality was not significant for Cohen’s d meta-analysis (adjusted effect size = −0.045, 95% CI: −1.36 to 1.27, p = 0.94) or odds ratio (adjusted effect size = −0.31, 95% CI: −1.06 to 0.42, p = 0.40) (Table S2). Moderators significantly explained the variance in Cohen’s d (QM = 69.7896, p < 0.0001) and odds ratio (QM = 191.318, p < 0.0001) meta-analyses. The meta-regression analysis identified which study characteristic categories moderated the effect size (Table S2). For Cohen’s d meta-analysis, the type of control category and the biological means collected category significantly moderated the effect size. The psychiatric control category (adjusted effect size = −0.19, 95% CI: −0.34 to −0.03, p = 0.016) and the cerebrospinal fluid category (adjusted effect size = −1.371, 95% CI: −2.4 to −0.33, p = 0.0093). For odds ratio meta-analysis, these categories moderated the effect size including type of suicidality, study design, and sample size. Specifically, the following moderators were significant: suicidal ideation (adjusted effect size = −0.10, 95% CI: −0.17 to −0.02, p = 0.0081), retrospective cohort (adjusted effect size = 3.24, 95% CI: 2.08 to 4.54, p < 0.0001), and small sample size (adjusted effect size = −2.69, 95% CI: −4.42 to −0.81, p = 0.0044).

Discussion

Our systematic review summarized the results of 102 studies and identified biological correlates of suicidality in youth. Our meta-analysis identified promising biological correlates of suicidality in youth (NLR, neutrophil, cortisol, and seropositive toxoplasmosis). These results will encourage further research on these inflammation and endocrine correlates. In addition, our meta-regression results revealed specific moderators of correlates of suicidality: type of suicidality, control, sample size, means collected, and sample size. This is critical for future studies of correlates of youth suicidality as the design of the studies would need to factor these moderators.

This systematic review of children and adolescents is valuable since suicide is the third leading cause of death in this age group,1 and this age group also undergoes significant physiological changes during development that differ from adults,9 which may lead to differences in the biological correlates associated with suicidality. Furthermore, youth are at increased risk for medication non-adherence, which increases the risk for suicide.130 Previous literature reviews linking biological correlates of suicidality in children and adolescents are limited, including two systematic reviews11 and one scoping review.10 Herein, 102 studies for peripheral biological correlates of suicidality were identified. The biological correlates were grouped into categories as noted in the STAR Methods section. Similar to other reviews,10,14 studies investigating genes, stress response system, serotonin system, immune system, and inflammatory markers were the most common types of biomarkers that were encountered.

This systematic review revealed the need for better documentation of the demographics and the inclusion of more diverse demographics in the sample. Many of the studies (n = 47, 46.1%) did not report the race and/or ethnicity of their samples. White/Caucasian individuals were the most studied group (n = 42, 41.2%), followed by Black individuals (n = 26, 25.5%). Furthermore, many of the studies grouped male and female data together, which may mask the results for meta-analysis as there are sex differences in biological correlates of suicidality.131,132 In addition, the systematic review found widespread use of the case-control (n = 67, 65.7%) study design. This raises concerns about selection and information biases, potentially confounding the findings.133,134

Although previous meta-analyses of biological correlates of suicidality have been conducted in the general population, these studies did not partition the meta-analyses for children and adolescents from adults.19,20,21,26 The current meta-analysis identified multiple biological correlates for suicidality (Figure 2 and Table S1), in the immune system, stress response system, and the infection serology categories. The current observations will inform future studies on identifying and establishing biomarkers with analytical validity and clinical utility.4

For the immune system, we identified multiple biological correlates associated with suicidality (complete blood count markers [CBC]; neutrophil, and NLR). NLR as a correlate of suicidality aligns with adult studies, which also noted that NLR was a better correlate than platelet to lymphocyte ratio (PLR).135,136 This could explain why PLR and platelet counts only trended significance in our analysis. Of note, for adults, the severity of suicide attempts was associated with PLR and platelet counts, which could not be factored into the results.137 For adult studies, studies were inconsistent on neutrophil counts.135,137 Additionally, we could not assess sex differences in the CBC and inflammatory ratio, which is a limitation as sex-based CBC differences in psychiatric disorders have been observed previously.138

The inflammation markers—tumor necrosis factor alpha (TNF-α) and cytokines (interleukin-6 [IL-6], IL-1β, interferon gamma [IFN-γ], IL-4), did not associate significantly with suicidality. In contrast, an adult meta-analysis of TNF-α, IL-6, and IL-1β observed a significant association of IL-6 with suicidality. This difference in IL-6 results between adults and youth could be due to the correlation of puberty and age with IL-6139 or the limited number of studies in this meta-analysis. In adult psychiatric samples, inflammation markers emerge as promising biomarkers of suicidality in adults, given the growing evidence of a dysregulated immune system in the pathophysiology of suicidality.140 Proposed mechanisms of behavioral changes from dysregulated immune systems include impairments of the kynurenine pathway of tryptophan catabolism, changes in monoamine metabolism, and increased activation of the HPA axis.141 Further research is necessary to validate these findings in younger populations.

Lipid levels have previously been associated with suicidality, with a proposed mechanism of altering the neuronal membranes, which affect inflammation and serotonergic transmission.142 While adult meta-analysis shows a link between lipid levels and suicidality,143 this study found that high-density lipoprotein (HDL) and low-density lipoprotein (LDL) did not significantly associate with suicidality in youth. This difference could be due to changes in lipid levels during puberty.144

Cortisol was also identified as a promising correlate for suicidality, though sensitivity analyses did not reveal a significant association. The discrepancy in the results could be due to the differences in the collection methods of cortisol. Adult studies show that dysregulation of the stress response system is associated with suicidality,6 with a meta-analysis showing a significant association of morning cortisol with suicidality.21 One of the extracted studies did not show a correlation between baseline cortisol and suicide attempt,56 but the salivary cortisol samples were collected from a time range of morning to afternoon. Toxoplasmosis seropositivity was significantly associated with suicidality in youth but was not significant in the sensitivity analysis. A meta-analysis of toxoplasmosis seropositivity was associated with suicidality in adult studies, but this was focused on suicidal behavior, and not suicidal ideation, which can explain the discrepancy in results.145

Meta-regression analysis identified study characteristics that affect the effect size of the biological correlates of suicidality (Table S2). We identified that the type of suicidality, type of control, sample size, study design, and how the biological samples were collected serve as significant moderators. These results emphasize the importance of considering these study characteristics when designing future studies of biomarkers of suicidality in adolescents. Of note, the way suicidality is measured is quite variable which also affected the analysis. While suicide attempts were generally classified as a binary outcome, suicidal thoughts were reported either as categorical77 by clinician assessment or via continuous data by validated scales such as the Beck Scale for Suicidal Ideation102 and the Suicidal Ideation Questionnaire.58

Another study characteristic that serves as a moderator is the type of control group used. Some studies include healthy controls,75 whereas others use psychiatric controls.38,45 Meta-regression with Cohen’s d effect size showed that psychiatric controls significantly decreased the overall effect size of biomarkers of suicidality. This finding emphasizes the inclusion of psychiatric controls when analyzing biological correlates. Although healthy controls provide important insights, using psychiatric controls may also provide a more precise assessment of the correlation between biomarkers and suicidality, as it helps distinguish the effects of suicidality from those of underlying psychiatric disorders, which are known to increase suicide risk.146

Another category that influences the relationship between biological correlates and suicidality is the method utilized for collecting the biological correlate. The meta-regression specifically noted that compared to collecting samples by blood, collecting samples by cerebrospinal fluid affects the relationship, which aligns with research in adult studies.5 A meta-analysis of adult suicidality and BDNF19 showed differences between BDNF collected from serum and plasma. Although the meta-regression did not find collection from serum or plasma to be a moderator, further investigation with specific correlates is warranted.

The methodological variability and limited number of studies underscore the need for more standardized research on biological correlates of suicidality. The meta-regression findings emphasized key moderators for biomarkers associated with suicidality, including the type of suicidality, type of control, means that the biological samples are collected, sample size, and study design. Improved documentation and standardization of demographics, study design, and sex-specific data are critical for enhancing the generalizability and consistency of suicide biomarker research in youth.

In conclusion, this systematic review and meta-analysis add valuable insights into the biological underpinnings of suicidality in children and adolescents. A comprehensive overview of 102 studies reveals how distinct biological correlates relate to suicidality in youth. Although we did not identify a biomarker of suicidality in youth, we identified promising biological correlates of suicidality, mainly related to immune function and the stress response system, warranting further research. This study identified peripheral markers with potential clinical utility in screening for suicidality in youth. Future studies should explore the temporal relationship of suicide-related variables with clinical features. As part of our ongoing efforts, our research group is currently conducting a longitudinal study to identify whether suicidality correlates with immune markers.147

Limitations of the study

A limitation is the exclusion of non-English language papers, which may have led to the omission of relevant studies. The publication bias also highlights the need for a more comprehensive search strategy, including expanded use of gray literature and unpublished studies. This paper was also narrow in the scope of looking at peripheral biological correlates and suicidality. Future research could investigate neural biological correlates and examine non-suicidal self-injury.

Another limitation is the limited number of studies, with individual biomarkers ranging from 3 to 9 results. Some of the meta-analyses may be underpowered to show the significance of biological correlates of suicide or publication bias, including funnel plot symmetry. Further replication studies are needed to establish these correlates as biomarkers of suicidality for adolescents. Some studies produced multiple results, but this effect is factored in the multi-level meta-analysis.

The multi-level meta-analysis accounts for variation across specific biomarkers, but a limitation is the effects of the moderators may be masked by biological correlates that indirectly relate to suicidality. Given that many biological correlates had both negative and positive effect sizes, the factor of biological correlates with indirect relation to suicidality could not be addressed in the framework. In addition, it would be ideal to conduct a meta-regression analysis of individual biomarkers, but due to the small number of studies for each individual biomarker, these meta-regression results would be limited and underpowered. In addition, we could not determine the time elapsed since the suicide attempt and the measurement of the biological correlate, which could impact the strength of the association.

Resource availability

Lead contact

Requests for further information and resources should be directed to and will be fulfilled by the lead contact, Dr. Madhukar H. Trivedi (Madhukar.trivedi@utsouthwestern.edu).

Materials availability

This study did not generate new unique reagents.

Data and code availability

  • This study is a systematic review and meta-analysis. The data analyzed in this study were extracted from previously published studies and publicly available resources. Raw data used for meta-analysis is available upon request.

  • The meta-analysis was conducted in R using the standard function available in the metafor package, and no custom code was developed. As a result, code sharing is not necessary for reproducing our results.

  • Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.

Acknowledgments

The authors wish to thank Hayley Aramburu and Srividya Vasu for administrative assistance. Thomas Pak is a psychiatry resident in “Translational Research Activities in Neuropsychiatry (TRAIN)” research training program funded by the National Institute of Mental Health (R25 MH101078; principal investigator – Dr. Madhukar H. Trivedi). The content is solely the authors’ responsibility and does not necessarily represent the official views of the National Institute of Health.

Author contributions

T.K.P., conceptualization, writing – original draft preparation, investigation, and data curation; E.R.A., conceptualization, project administration, writing – original draft preparation, and investigation; T.C., formal analysis and writing – review and editing; L.J., writing – review and editing, investigation, and data curation; Z.F., writing – review and editing, and investigation; A.N., writing – review and editing, and data curation; G.E., conceptualization, writing – review and editing; M.H.T., conceptualization, funding acquisition, project administration, resources, supervision, and writing – review and editing.

Declaration of interests

T.K.P., E.R.A., D.C., L.J., Z.F., and A.N. report no conflicts of interest. G.E. is a consultant for Lundbeck and Neuronetics. G.E. receives research support from American Foundation for Suicide Prevention (AFSP), Janssen Pharmaceuticals, Janssen Research & Development, the National Institutes of Health, Patient-Centered Outcomes Research Institute (PCORI), and the State of Texas. M.H.T. has received research funding from NIMH, NIDA, NCATS, the American Foundation for Suicide Prevention, the Patient-Centered Outcomes Research Institute, and the Blue Cross Blue Shield of Texas. He has served as a consultant or advisor for ACADIA PHARMACEUTICALS INC., Akili Interactive, ALKERMES INC (Pub Steering Comm-ALKS5461), Allergan Sales LLC, Alto Neuroscience, Inc. , Applied Clinical Intelligence, LLC (ACI), Axome Therapeutics, Boehringer Ingelheim, Engage Health Media, Gh Research, GreenLight VitalSign6, Inc., Heading Health, Inc., Health Care Global Village, Janssen – Cilag.SA, Janssen Research and Development, LLC (Adv Committee Esketamine), Janssen Research and Development, LLC (panel for study design for MDD relapse), Janssen - ORBIT, Legion Health, Jazz Pharmaceuticals, LUNDBECK RESEARCH U.S.A, Medscape, LLC, Merck Sharp & Dohme Corp., Mind Medicine (MindMed) Inc., Myriad Neuroscience, Neurocrine Biosciences Inc, Navitor, Pharmaceuticals, Inc., Noema Pharma AG, Orexo US Inc., Otsuka Pharmaceutical Development & Commercialization, Inc. (PsychU, MDD Section Advisor), Otsuka America Pharmaceutical, Inc. (MDD expert), Pax Neuroscience, Perception Neuroscience Holdings, Inc., Pharmerit International, LP, Policy Analysis Inc., Sage, Therapeutics, Rexahn Pharmaceuticals, Inc., Sage Therapeutics, Signant Health, SK Life Science, Inc., Takeda Development Center Americas, Inc., The Baldwin Group, Inc., and Titan Pharmaceuticals, Inc. M.H.T. also received editorial compensation from Oxford University Press. Disclaimer: The Intellectual Property of VitalSign6 belongs to the University of Texas Southwestern Medical Center (Principal Investigator, M.H.T.) and is now licensed to GLVS6 for future distribution.

STAR★Methods

Key resources table

REAGENT or RESOURCE SOURCE IDENTIFIER
Software and algorithms

Covidence https://www.covidence.org/
R with metafor package https://www.r-project.org/
Prospero https://www.crd.york.ac.uk/prospero/
Microsoft Excel https://www.microsoft.com/en-us/microsoft-365/excel

Other

Data published/compiled from the literature PubMed https://www.ncbi.nlm.nih.gov/pubmed
Data published/compiled from the literature the Cochrane Library https://www.cochranelibrary.com/
Data published/compiled from the literature Embase https://www.embase.com
Data published/compiled from the literature PsycINFO https://www.apa.org/pubs/databases/psycinfo

Experimental model and study participant details

Inclusion and exclusion criteria

Studies were eligible for inclusion if they were cohort, cross-sectional, case-series, or randomized controlled studies, and if they provided empirical data on peripheral biological correlates of suicidality in children and adolescents. The population of interest includes children (6-12 years of age) and adolescents (13-18 years of age) as defined by Medical Subject Headings (MESH) produced by the National Library of Medicine. Some studies did not include the age range but were included only if they explicitly noted they included children or adolescents. If a study had the age range of interest but included patients outside the age range, there were two conditions to keep them: 1) the study included a subgroup analysis for the age range of interest, or 2) the study’s age range was a maximum of 30 years old and included adolescents. We used 30 years of age as a cut-off to include as many adolescent studies since adolescence can be defined as the onset of puberty to ending in the mid-20s.9 To control the age-related variability, the age groups were categorized as children, adolescents, and young adults. Reviews, editorials, unpublished literature, book chapters, conference proceedings, abstracts (without an associated paper), studies not in English, animal studies, and in vitro studies were excluded.

Method details

Procedures

After consulting with a research librarian experienced in systematic reviews, a comprehensive search strategy was developed, and the study was registered in PROSPERO (CRD42023417128). Any deviations to the protocol are noted in the supplemental information (Document S1). We adhered to PRISMA guidelines (Table S3).

Four databases (PubMed, the Cochrane Library, Embase, and PsycINFO) were utilized to identify articles published from the earliest date of the respective database to April 2023. Search terms were chosen to identify peripheral biological correlates of suicidality and self-harm behavior in children and adolescents. The full details of the search strings are provided in (Table S4). In addition to database searches, manual searches of reference lists from relevant review articles were conducted to ensure comprehensive coverage.

The web-based systematic review software Covidence was utilized to assist with screening, including the removal of duplicate articles. Initial screening based on titles and abstracts was performed independently by two authors (TP, LJ), and full-text screenings were also performed independently by two authors (TP, LJ). Disagreements were resolved by a third author (EA). Data extraction was completed using a standardized template based on previous systematic reviews.10,148 The following data were extracted into categories: date of publication, sex, age, race and ethnicity, sample size, population source recruited, study design, biological correlate category, and types of suicidality. For race and ethnicity, the categories were based on the NIH general guidance for race and national origin.149 The biological correlate categories were determined from previous reviews10,17,150 and discussion among the authors. We categorized the biomarkers (Table 1) using the twelve categories: Cytokines, the endocrine system, gene expression, genes, the immune system, inflammatory markers, metabolism, infection serology, neuromodulators, neurotrophins, the serotonin system, and the stress response system. The “endocrine system” consists of hormones, including growth hormone28 and thyroid stimulating hormone.151 The “immune system” is a wide category, which includes “inflammatory markers” and “cytokines.” However, given their distinct roles in the immune system, we separated “inflammatory markers” and “cytokine” for an improved categorization framework of the biological correlates. Although these biomarkers work in concert during pathological processes and may have dependent or independent functions, we categorized them based on their biological types/classifications (cells vs. cytokines vs. acute phase proteins). Similarly, the “stress response system,” which includes cortisol, was separated from the “endocrine system,” and we had separate categories for “neuromodulators” and “serotonin system,” aligning with previous reviews.10,17 Table 1 includes the biological correlates and their corresponding biological categories. Table 1 also includes study numbers (not reference numbers) that we used to refer to specific studies in our figures. Types of suicidality were separated into three categories: suicide attempt, suicidal ideation, and individuals with suicidal ideation or attempts grouped together. The suicide attempt category included any self-harm attempt with any intent to die. The suicidal ideation category included any thoughts/plans to die by suicide. The third category was individuals with suicide attempt or suicidal ideation grouped into a single group. For example, one study grouped “MDD [major depressive disorder] with recent suicide behavior/ideation” into a single group.29 Of note, some individual articles looked at multiple biomarker categories and multiple types of suicidality. For types of control, it was noted if the study compared suicidality with groups that are psychiatric controls (such as MDD with suicidality vs MDD without suicidality29), specific population controls (such as juvenile detainees with suicidal ideation vs juvenile detainees without suicidal ideation50), and healthy controls.54

The quality of the included studies was independently assessed by two authors (TP, ZF) using the Joanna Briggs Institute (JBI) Critical Appraisal tools evidence.152 The JBI tool provides a checklist to evaluate the methodological quality based on the study design of the case series, case-control, and cohort studies. Articles were of excellent quality (scored 100%), good quality (scored 70%-99% of the checklist), fair quality (scored 50%-70% of the checklist), and poor quality (scored 0%-50% of the checklist). The cutoffs were determined from other research articles153,154 using JBI tools. Deviations from the original registered PROSPERO protocol are noted in the supplemental information.

Quantification and statistical analysis

All statistical analyses were performed using “R” with the “metafor” package. Figures were created using “R” and tables were created using Microsoft Excel. The meta-analysis of peripheral biological correlates of suicidality was conducted using effect sizes for Cohen’s d or effect sizes for the natural log of odds ratio. To improve the variance estimate, an estimation method of Restricted Maximum Likelihood was used.155

To calculate Cohen’s d for meta-analysis, the studies used reported 1) mean and standard deviation of biological correlate levels, 2) median, minimum, and maximum of biological correlate and calculations based on Wan et al.,156 3) odds ratios derived from dichotomization of continuous biological correlate and converted to Cohen’s d based on calculations from Chinn157; (we did not convert odds ratios that derived from non-continuous biological correlates), and 4) Pearson correlations of biological correlates converted to Cohen’s d based on calculations from Mathur and VanderWeele.158 With data that reported median and IQR, the 1st quartile and 3rd quartile range were approximated, assuming a normal distribution. For the interpretation of effect size (Cohen’s d), a value of 0.2 was considered small, 0.5 was considered moderate, and 0.8 was considered large.

A single-level mixed-effect meta-analysis was conducted for individual biomarkers from at least three studies.129 A separate meta-analysis based on the type of suicidality was not conducted as that would have limited the number of individual biomarkers. Instead, a sensitivity analysis was conducted that accounted for the type of suicidality, and multiple results were from the same study. This analysis was conducted on the four significant biological correlates after Bonferroni correction. Sensitivity analysis was conducted with a multi-level mixed-effects meta-analysis to account for multiple measurements per study and factored in the fixed effect of type of suicidality.159 To assess the certainty of the meta-analysis of the individual biomarker, the Grades of Recommendation, Assessment, Development, and Evaluation (GRADE) guideline was used.160 These results are shown in Table S1.

To examine study characteristics that moderate the strength of biological correlate to suicidality, a multi-level meta-analysis and meta-regression of all the studies with available effect sizes was conducted. The meta-analysis was conducted using effect sizes for Cohen’s d or effect sizes for the natural log of odds ratio. The following moderators were studied: study design, sex, age category, sample size category, type of controls, means collected, and type of suicidality. Many studies did not specify the means the biological samples were collected. In addition, hematological measures were from whole blood samples, lipids were measured using serum samples, and cytokines were collected from whole blood samples or plasma.

An Egger’s test was conducted to assess publication bias in the meta-analysis. The test measures asymmetry in a funnel plot, which indicates publication bias. The funnel plots were also visually inspected to assess for asymmetry.

Additional resources

Amendments to protocol

The Supplementary Information documents the changes to the registered PROSPERO protocol (CRD42023417128).

Published: March 25, 2025

Footnotes

Supplemental information can be found online at https://doi.org/10.1016/j.isci.2025.112290.

Supplemental information

Document S1. Figures S1 and S2, Tables S1–S4, and Data S1
mmc1.pdf (696.5KB, pdf)

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

Document S1. Figures S1 and S2, Tables S1–S4, and Data S1
mmc1.pdf (696.5KB, pdf)

Data Availability Statement

  • This study is a systematic review and meta-analysis. The data analyzed in this study were extracted from previously published studies and publicly available resources. Raw data used for meta-analysis is available upon request.

  • The meta-analysis was conducted in R using the standard function available in the metafor package, and no custom code was developed. As a result, code sharing is not necessary for reproducing our results.

  • Any additional information required to reanalyze the data reported in this paper is available from the lead contact upon request.


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