Abstract
Background
Biological sex plays a crucial role in the pathophysiology of morbidities related to preterm birth. Studies specifically investigating the role of biological sex in neonatal kidney disease are lacking. This study aimed to determine the association between biological sex and kidney outcomes in preterm infants. We hypothesized that male infants would be more likely to have poor kidney outcomes, including acute kidney injury (AKI), hypertension and chronic kidney disease. Given data on the relationship between sex, AKI and lung disease, we also evaluated lung disease as a secondary outcome.
Methods
Retrospective analysis of the Preterm Erythropoietin Neuroprotection Trial data. For adjusted models, we used covariates associated with adverse outcomes a priori (gestational age, SGA status) and a lasso regression. AKI was defined using creatinine only neonatal modified KDIGO criteria and bronchopulmonary dysplasia (BPD) by Neonatal Research Network criteria.
Results
Of the 923 infants included, 479 (51.8%) were male. AKI was more common among males (aOR 1.31, 95%CI 1.00–1.74). Hypertension was nearly twice as common in males (66.4% vs. 51.5%, p < 0.001) and remained after adjustment (aOR 1.91, 95%CI 1.32–2.77). Severe BPD was more common in males (33.4% vs. 24.3%, p = 0.0024), which persisted after adjustment (aOR 1.65, 95%CI 1.22–2.23). This association was not fully mitigated by AKI exposure.
Conclusions
We describe differences in kidney outcomes and BPD by sex. Male sex is associated with an increased risk of AKI and hypertension. Research efforts focused on the mechanisms underlying sex-specific differences are needed for the identification of novel therapies that benefit both sexes.
Graphical abstract
A higher-resolution version of the Graphical abstract is available as Supplementary information
Supplementary Information
The online version contains supplementary material available at 10.1007/s00467-025-07131-3.
Keywords: Acute kidney injury, Chronic lung disease, Chronic kidney disease, Hypertension, Prematurity
Introduction
Sex is an important biological variable in the development of multiple diseases in preterm neonates. The evidence of the role of sex as a biological variable in pathophysiology, disease outcome, and response to therapy in neonatal clinical studies continues to increase [1–3]. This includes the development of bronchopulmonary dysplasia (BPD) and intraventricular hemorrhage (IVH) [4, 5]. In older populations, sex has been found to be an important biological variable for kidney-specific outcomes such as acute kidney injury (AKI) and progression to chronic kidney disease [6–11]. For example, female sex is protective in ischemia–reperfusion AKI and deleterious in some forms of nephrotoxin-mediated AKI. Possible explanations for these findings include hormonal differences (which may be mitigated in the perinatal period by circulating maternal hormones) and chromosomal differences (XX vs. XY complement).
Studies investigating the role of biological sex in neonatal kidney diseases, including AKI, are lacking. Most clinical studies do not report data disaggregated by sex. The National Institutes of Health published a call to action for the inclusion of both sexes in pre-clinical and clinical research to increase the transparency of studies [6]. Determining if sex-based differences exist and the mechanism of protective sex biases would allow researchers to identify therapeutic targets that benefit both sexes and has recently been highlighted as an important area of further research [12].
We performed a secondary analysis of the Preterm Erythropoietin Neuroprotection (PENUT) Trial, a multicenter clinical trial that randomized extremely preterm neonates to receive erythropoietin or placebo for neuroprotection. The PENUT study captured robust data including short and long-term kidney-related outcomes [13–15]. While several secondary analyses of the PENUT study have previously reported individual morbidities including AKI, BPD, and late-onset infections, this present study extends this body of work by evaluating sex as a primary exposure across multiple organ systems [14, 16, 17]. We sought to determine the association between biological sex and short-term and long-term kidney outcomes in preterm infants. This integrated approach allows assessment of the association between biologic sex and outcomes within the same cohort with harmonized methods. We hypothesized that male infants would be more likely to have poor short-term and long-term kidney outcomes. Given existing studies that have found a strong relationship between sex and AKI as well as AKI and BPD, we also hypothesized that AKI would mitigate the long-established relationship between sex and BPD [18–20].
Methods
Study design
The PENUT Trial was a Phase III randomized, placebo-controlled, double-blind trial of erythropoietin in extremely preterm neonates in 19 academic centers in the United States [15]. The inclusion criteria were (1) gestational age 24 0/7 and 27 6/7 weeks’, (2) enrollment < 24 h of age, and (3) arterial or venous access. Exclusion criteria included (1) major life-threatening anomalies, (2) hematologic crises (e.g., disseminated intravascular coagulation, hemolysis), (3) hematocrit > 65%, (4) hydrops fetalis, and (5) congenital infection.
The Recombinant Erythropoietin for Protection of Infant Renal Disease (REPAIReD) Study was an ancillary study to assess longitudinal kidney function. The Institutional Review Board (IRB) at the University of Washington served as the central IRB and each center involved in the study received approval from their IRB or Human Research Ethics Committee. Consent was obtained from the parent or legal guardian. The PENUT trial is registered with ClinicalTrials.org (NCT01378273). This was a post hoc analysis performed in March 2025 and follows the STROBE reporting guidelines for observational studies [21].
Definitions of exposure and key outcomes
Sex was determined for each study participant by chart review of the medical record, with this assigned as either male, female, or unknown. We determined AKI stage using Kidney Diseases Improving Global Outcomes definition [22]. Severe AKI was defined as Stage 2 or 3. We did not use urine output criteria to determine AKI. For AKI determination, we used a baseline of the lowest creatinine at any point starting on postnatal day 2. Individual creatinine measurements had to be > 0.5 mg/dL to be considered in the AKI calculation. As creatinine levels on postnatal days 0–2 may represent maternal levels, we began AKI calculation on postnatal day 3 [13]. We note that our AKI definition used all creatinine values collected per protocol, which differs from Turner et al., who excluded infants without daily creatinine available and applied slightly different criteria in AKI definitions [17].
We determined BPD severity by respiratory support at 36 weeks’ post-menstrual. We evaluated a composite outcome of severe BPD or death, defined using the most recent Neonatal Research Network definitions [23]. We used severe BPD (Neonatal Research Network defined Grade 2 or Grade 3 BPD) to focus on preterm neonates most at risk for long-term pulmonary disease [23, 24]. We used a composite of severe BPD/death given that they are competing outcomes. Infants without complete respiratory data or without creatinine measurements were excluded, resulting in a slightly narrower analytic cohort than previous BPD studies in this cohort [16].
For two-year outcomes, we included those with estimated glomerular filtration rate (eGFR), urine albumin/creatinine ratio (ACR), or blood pressure (BP) measured at the two-year visit. We excluded those with death prior to 24 months, no follow-up, no blood or urine samples obtained, nor BP measured. Of the 780 infants with 2-year data, 565 had at least one kidney outcome measure including 348 with eGFR, 435 with urine testing, and 289 with BP measurement [25]. We evaluated three a priori designated kidney outcomes at two years [20, 27]: (1) eGFR < 90 ml/min/1.73 m2 using the CKID U25 equation (including sCr and cystatin C) [28, 29], (2) albuminuria, defined as ACR > 30 mg albumin/g creatinine [15], and (3) systolic or diastolic BP > 90th percentile for age and sex based on 2017 AAP guidelines (i.e., hypertension) [27, 30]. Urine samples were collected using a bag specimen or a cotton ball in the diaper. BP readings were obtained using a Briggs Mabic Healthcare Manual Sphygmomanometer with an appropriately sized BP cuff, measured twice, with a five-minute interval between readings. The lowest systolic BP and diastolic BP were recorded [20, 21].
Statistical analysis
Categorical variables were analyzed by proportional differences with the chi-square test or the Fisher exact test. T-test and Wilcoxon rank-sum test were used to compare continuous and ordinal variables, respectively. Odds ratios (OR) and associated 95% confidence intervals (CIs) for the association between sex and outcomes of interest were estimated from unconditional logistic regression models. Given the existing studies from our group and others that have found a strong relationship between sex and AKI as well as AKI and BPD, and a concern that AKI may play a role in this relationship between sex and the composite outcome, we then stratified our multivariable analysis by AKI status [18–20].
Multivariable logistic regression was performed using a lasso regression model. We created three multivariable models in keeping with existing literature evaluating sex differences in neonatal outcomes. Our models included one adjusted for gestational age, one by gestational age and small for gestational age (SGA) status, and then a final model including an a priori list of variables (gestational age, small for gestational age (SGA) status, maternal race, pre-eclampsia, intubation, surfactant use, and chest compressions during resuscitation) based on the existing literature was used to account for potential confounding variables. All findings are reported as adjusted ORs with 95%CIs. In all analyses, a p-value < 0.05 was considered statistically significant. Analysis was performed using SAS version 9.4 (Statistical Analysis Software Institute Inc., Cary, NC, USA).
Results
Of the 941 neonates who met inclusion criteria for PENUT, 923 neonates were eligible for this analysis. The reasons for exclusion of the 18 neonates included no treatment given in the parent trial (n = 4), the patient being incorrectly enrolled (n = 1) or death within the first two days of life (n = 13), all of which were exclusion criteria in the parent trial [15]. Of those included in this analysis, 51.9% (479) were male and 48.1% (444) were female (Table 1). The median birth weight was 800 g (IQR 668, 940), and 15.3% were SGA. Maternal comorbidities were common in the cohort, including multiple gestations (26.4%), pre-eclampsia (15.2%), and hypertension (7.5%). Antenatal steroids were received by 91.6% of mothers of infants in the study. Most infants (97.2%) required some delivery room resuscitation, with 81.0% intubated in the delivery room.
Table 1.
Baseline neonatal and maternal characteristics of cohort stratified by infant sex, male vs. female
| Characteristic | Overall (n = 923) | Male infants (n = 479) | Female infants (n = 444) | p-value* |
|---|---|---|---|---|
| Erythropoietin (%) | 454 (49.2) | 228 (47.6) | 226 (50.9) | 0.32 |
| Gestational age, weeks (%) | ||||
| 24 | 227 (24.6) | 121 (25.3) | 106 (23.9) | 0.94 |
| 25 | 242 (26.2) | 122 (25.5) | 120 (27.0) | |
| 26 | 220 (23.8) | 114 (23.8) | 106 (23.9) | |
| 27 | 234 (25.4) | 122 (25.5) | 112 (25.2) | |
| Median Birth weight, grams (IQR) | 800 (668–940) | 820 (690–953) | 770 (641–900) | < 0.0001 |
| Small size for gestational age (%) | 141 (15.3) | 65 (13.7) | 76 (17.2) | 0.14 |
| Median Apgar 1 min (IQR) | 4 (2–6) | 4 (2–6) | 4 (2–6) | 0.07 |
| Median Apgar 5 min (IQR) | 7 (5–8) | 7 (5–8) | 7 (5–8) | 0.02 |
| Delivery room resuscitation (%) | ||||
| Any | 896 (97.2) | 469 (97.9) | 427 (96.4) | 0.16 |
| Intubation | 748 (81.0) | 396 (82.7) | 352 (79.3) | 0.19 |
| Surfactant | 480 (52.0) | 254 (53.0) | 226 (50.9) | 0.52 |
| Chest compressions | 72 (7.8) | 34 (7.1) | 38 (8.6) | 0.41 |
| Resuscitation drugs | 32 (3.4) | 19 (3.9) | 13 (2.9) | 0.39 |
| Maternal characteristics (%) | ||||
| Multiple gestations | 243 (26.4) | 126 (26.3) | 117 (26.4) | 0.99 |
| Diabetes | 48 (5.2) | 27 (5.6) | 21 (4.7) | 0.54 |
| Hypertension | 70 (7.5) | 29 (6.0) | 41 (9.2) | 0.07 |
| Pre-eclampsia | 140 (15.2) | 58 (12.1) | 82 (18.5) | 0.007 |
| Antenatal steroids | 831 (91.6) | 430 (91.9) | 401 (91.3) | 0.77 |
| Maternal race (%) | ||||
| Black | 239 (25.9) | 115 (24.0) | 124 (27.9) | 0.38 |
| White | 604 (65.4) | 320 (66.8) | 284 (64.0) | |
| Other/Unknown | 80 (8.7) | 44 (9.2) | 36 (8.1) | |
| Maternal ethnicity (%) | ||||
| Hispanic or Latino | 197 (21.2) | 105 (21.6) | 92 (20.7) | 0.83 |
| Not Hispanic or Latino | 715 (77.5) | 369 (77.0) | 346 (77.9) | |
| Unknown | 11 (1.2) | 5 (1.0) | 6 (1.4) | |
*Estimated from a chi-square or Wilcoxon Rank sums test for categorical and continuous variables, respectively
Male infants had higher median birth weights (820 g vs. 770 g, p < 0.0001), while female infants had higher 5-min Apgar scores (p = 0.02). A higher proportion of mothers of female infants had pre-eclampsia (18.5% vs. 12.1%, p = 0.007). Other maternal characteristics, including race, ethnicity, and maternal co-morbidities, did not differ significantly by infant sex.
Short-term outcomes
Rates of AKI of any stage were higher among males (43.8%) than females (37.4%, p = 0.046); however, there was no difference in the rate of severe AKI between males and females (Table 2). In fully adjusted models, a similar association was observed between biologic sex and AKI, though the association was only significant for any stage AKI; specifically, males had 30% higher odds of any stage AKI (OR 1.31, 95%CI 1.00–1.74). The adjusted association was not seen for severe AKI (OR 1.33, 95%CI 0.95–1.85) (Table 3).
Table 2.
In-hospital and long-term outcomes stratified by infant sex, male vs. female
| In-hospital outcomes | Overall (n = 923) | Male infants (n = 479) | Female infants (n = 444) | p-values* |
|---|---|---|---|---|
| Short-term, n (%) | ||||
| AKI | ||||
| Any stage | 376 (40.7) | 210 (43.8) | 166 (37.4) | 0.0462 |
| Stage 2/3 | 196 (21.2) | 112 (23.4) | 84 (18.9) | 0.0976 |
| PDA (treated) | 389 (42.0) | 212 (43.9) | 177 (39.9) | 0.2145 |
| Death | 100 (10.8) | 56 (11.7) | 44 (9.9) | 0.3844 |
| Respiratory, n (%) | ||||
| Mechanical Ventilation at 14 postnatal days | 480 (52.0) | 260 (54.3) | 220 (49.5) | 0.1507 |
| Severe BPD | 268 (29.0) | 160 (33.4) | 108 (24.3) | 0.0024 |
| Death or Severe BPD | 341 (36.9) | 199 (41.5) | 142 (32.0) | 0.0026 |
| Respiratory status at discharge | ||||
| Room air | 510 (55.7) | 264 (55.2) | 246 (56.2) | 0.3131 |
| Nasal cannula | 306 (33.2) | 154 (32.2) | 152 (34.7) | |
| Tracheostomy | 14 (1.5) | 7 (1.5) | 7 (1.6) | |
| Ventilated | 86 (9.3) | 53 (11.1) | 33 (7.5) | |
| IVH | ||||
| Severe IVH | 122 (13.1) | 68 (14.0) | 64 (12.2) | 0.3619 |
| PVL | 55 (5.9) | 32 (6.6) | 23 (5.2) | 0.3360 |
| ROP | 60 (6.5) | 33 (6.8) | 27 (6.1) | 0.6187 |
| NEC | 99 (10.7) | 52 (10.7) | 47 (10.6) | 0.9039 |
| Median Length of stay (IQR) | 93 (71–114) | 93 (71–118) | 92 (71–111) | 0.3404 |
| Long-term | ||||
| Low eGFR | 54/336 (16.1) | 28/175 (16.0) | 26/161 (16.1) | 0.9704 |
| Albuminuria | 155/435 (35.6) | 74/233 (31.8) | 81/202 (40.1) | 0.0701 |
| Hypertension | 290/492 (58.9) | 166/251 (66.1) | 124/241 (51.5) | 0.0009 |
*Estimated from a chi-square or Wilcoxon rank sums for categorical and continuous variables, respectively
AKI, Acute Kidney Injury; BPD, Bronchopulmonary Dysplasia; eGFR, estimated Glomerular Filtration Rate; IQR, Interquartile Range; IVH, Intraventricular Hemorrhage; NEC, Necrotizing Enterocolitis; PDA, Patent Ductus Arteriosis; PVL, Periventricular Leukomalacia; ROP, Retinopathy of Prematurity
Table 3.
Multivariable analysis of kidney outcomes in male and female infants
| Variable | Male infant | Female infant | Model 11,2 | Model 2 | Model 3 | Model 4 |
|---|---|---|---|---|---|---|
| Kidney outcomes | ||||||
| AKI | ||||||
| Any stage | 210/479 (43.8) | 166/444 (37.4) |
1.31 (1.00–1.70) |
1.32 (1.01–1.74) |
1.34 (1.01–1.76) |
1.31 (1.00–1.74) |
| Stage 2/3 | 112/479 (23.4) | 84/444 (18.9) |
1.31 (0.95–1.80) |
1.31 (0.95–1.81) |
1.35 (0.97–1.88) |
1.33 (0.95–1.85) |
| Low eGFR |
28/175 (16.0) |
26/161 (16.1) |
0.99 (0.55–1.77) |
0.98 (0.54–1.76) |
0.98 (0.54–1.79) |
1.07 (0.58–1.98) |
| Albuminuria |
74/233 (31.8) |
81/202 (40.1) |
0.69 (0.46–1.02) |
0.68 (0.46–1.01) |
0.70 (0.47–1.04) |
0.68 (0.45–1.04) |
| Hypertension | 166/251 (66.1) | 124/241 (51.5) |
1.84 (1.28–2.65) |
1.86 (1.29–2.69) |
1.86 (1.29–2.69) |
1.91 (1.32–2.77) |
| Pulmonary outcomes | ||||||
| Death/Severe BPD |
199 (41.5) |
142 (32.0) |
1.52 (1.16–1.99) |
1.54 (1.17–2.02) |
1.60 (1.21–2.11) |
1.65 (1.24–2.19) |
| Death |
56 (11.7) |
44 (9.9) |
1.19 (0.78–1.80) |
1.20 (0.78–1.82) |
1.26 (0.82–1.94) |
1.31 (0.84–2.02) |
| Severe BPD |
160 (33.4) |
108 (24.3) |
1.56 (1.17–2.08) |
1.56 (1.17–2.09) |
1.62 (1.20–2.17) |
1.65 (1.22–2.23) |
| Pulmonary outcomes stratified by AKI status | ||||||
| No AKI | 269 | 278 | ||||
| Death/Severe BPD |
100 (37.2) |
78 (28.1) |
1.52 (1.06–2.18) |
1.56 (1.09–2.25) |
1.59 (1.10–2.30) |
1.70 (1.17–2.48) |
| Death |
27 (10.0) |
22 (7.9) |
1.27 (0.71–2.29) |
1.32 (0.73–2.39) |
1.33 (0.73–2.42) |
1.38 (0.75–2.55) |
| Severe BPD |
76 (28.3) |
58 (20.9) |
1.51 (1.02–2.23) |
1.53 (1.03–2.26) |
1.54 (1.04–2.29) |
1.62 (1.08–2.42) |
| AKI | 210 | 166 | ||||
| Death/Severe BPD |
99 (47.1) |
64 (38.6) |
1.44 (0.95–2.17) |
1.45 (0.95–2.20) |
1.59 (1.04–2.44) |
1.59 (1.03–2.45) |
| Death |
29 (13.8) |
22 (13.3) |
1.04 (0.57–1.89) |
1.05 (0.57–1.91) |
1.17 (0.64–2.15) |
1.21 (0.65–2.26) |
| Severe BPD |
84 (40.0) |
50 (30.1) |
1.57 (1.02–2.41) |
1.57 (1.02–2.42) |
1.68 (1.08–2.61) |
1.68 (1.07–2.63) |
1Estimated from a logistic regression
2Models: 1. Unadjusted OR (95%CI), 2. Adjusted for gestational age, 3. Adjusted for gestational age and SGA status, 4. Adjusted for gestational age, SGA status, maternal race, pre-eclampsia, and use intubation, surfactant use, and chest compressions during resuscitation
Non-kidney short-term complications such as treated patent ductus arteriosus, need for mechanical ventilation at 14 days, and respiratory support at discharge did not differ significantly by sex, though trends showed slightly higher rates of complications in males (Table 3). Rates of severe IVH, periventricular leukomalacia, retinopathy of prematurity, and necrotizing enterocolitis were comparable between groups, and length of hospital stay was nearly identical (median of approximately 93 days with an IQR of 71 to 114 days).
Long-term outcomes
The rates of low eGFR and albuminuria at two years were similar between sexes, with no differences in prevalences. Males had lower rates of albuminuria at 2 years (31.8% vs. 40.1%); however, this finding was not significant in either unadjusted or adjusted models (Table 3).
Hypertension was significantly more common in male infants (66.1%) compared to females (51.5%, p < 0.001). This association remained in the multivariable models, with males being almost twice as likely to have hypertension at two years of age than females (aOR 1.9, 95%CI 1.32–2.77) (Table 3).
Evaluating the relationship between AKI and BPD
Among the cohort, 100 (10.8%) died and 268 (29.0%) had severe BPD diagnosed at 36 weeks’ postmenstrual age, with 341 (36.9%) meeting the composite outcome of death or severe BPD (Table 2). While overall mortality was similar between male and female infants (11.7% vs. 9.9%, p = 0.38), male infants had a higher incidence of severe BPD at 33.4% compared to 24.3% in females (p = 0.0024). When combining death or severe BPD as a composite outcome, male infants had worse outcomes (41.5% vs. 32.0%, p = 0.0026), indicating a higher burden of severe respiratory morbidity among male preterm infants. In fully adjusted models, males were 65% more likely to experience the composite outcome of death or severe BPD (OR 1.65, 95%CI 1.24–2.19) and approximately 1.7 times more likely to develop severe BPD (OR 1.65, 95%CI 1.22–2.23) (Table 3). There was no difference by AKI status, in fully adjusted models; in particular, the association between male sex and severe BPD was OR 1.62 (95%CI 1.08–2.42) among neonates without AKI and 1.68 (95%CI 1.07–2.63) for neonates with AKI, and for the combined outcome of death and severe BPD was OR 1.70 (95%CI 1.17–2.48) among neonates without AKI and OR 1.59 (95%CI 1.03–2.45) for neonates with AKI (Table 3).
Discussion
In this large multicenter secondary analysis of preterm infants, we found that male infants exhibited higher rates of AKI and hypertension by 2 years of age. Although prior secondary analysis of the PENUT cohort have characterized individual morbidities, our findings extend this body of work by evaluating sex as the primary exposure using harmonized analytic methods across outcomes. Male sex remained independently associated with a greater risk of AKI during the neonatal period, and the relationship between male sex and respiratory outcomes persisted after stratifying by AKI status. Thus, our findings indicate a male disadvantage across respiratory and kidney morbidity in this extremely preterm cohort.
Our results align with a growing body of literature on sex-related disparities in neonatal outcomes [26]. Previous studies have repeatedly demonstrated that male preterm infants experience worse respiratory outcomes [1]. For example, a propensity-matched analysis found that extremely low birth weight males had a significantly higher incidence of respiratory distress syndrome and moderate-to-severe BPD after adjustment for confounders [11]. Likewise, male infants require more invasive respiratory support and are more likely to need home oxygen or tracheostomy when diagnosed with BPD [27]. We note that mothers of female infants were more likely to have pre-eclampsia, which has been found to have a maturational impact on the lungs of infants. This theoretically would lead to a decrease in lung disease in female infants and therefore a reduced rate of BPD — however, we adjusted for pre-eclampsia in our analysis and the finding that male infants had a higher rate of BPD persisted. Our finding of increased severe BPD and adverse composite outcomes in males is therefore consistent with this “male disadvantage” in neonatal respiratory disease.
Data about kidney outcomes are more limited but suggest a similar pattern of vulnerability [6]. Clinical reports have noted a male predominance among neonatal AKI cases [13, 14]. Experimental models support the idea that female sex is relatively protective: in an animal model of neonatal nephron loss, only male rats developed severe long-term kidney injury, leading the authors to conclude that male sex may be an independent risk factor for kidney disease [28]. These observations lend context to our finding that, even after adjusting for gestational age and other factors, male infants were at higher risk for AKI. We note that while several of our findings reached statistical significance, the absolute effect sizes were modest. These findings align with well-described male disadvantage in neonatal outcome but also suggest that sex, while biologically relevant, likely serves as one of multiple factors contributing to risk stratification.
Several findings from our study are novel. The most striking is the rate of hypertension among male infants at 2 years. While prematurity itself has been found to be a risk factor for childhood hypertension, few prior studies have reported sex differences in this outcome. In the PENUT cohort as a whole, nearly one-quarter of infants had systolic blood pressure above the 95th percentile by 2 years; our secondary analysis suggests that this burden falls disproportionately on males [25]. The mechanisms behind this sex-specific risk for hypertension remain unclear, but can be hypothesized to be in part due to differences in renal and cardiovascular development. Male neonates might be more vulnerable to nephron injury or have different renin–angiotensin and androgenic influences than females, leading to higher blood pressure [28]. Likewise, our finding of an adjusted association between male sex and AKI suggests a potential sex-based susceptibility. This may reflect sex differences in kidney maturation or response to injury; for instance, female preterm infants have been reported to have higher antioxidant enzyme activity than males, which could mitigate injury [2]. Regardless of mechanism, the clinical importance is clear: male preterm infants in our study not only fared worse in the neonatal intensive care unit but also carried more evidence of kidney and cardiovascular sequelae into early childhood.
Our study has notable strengths. The use of data from the PENUT/REPAIReD cohort has robust follow-up and prospectively collected data on respiratory, neurologic, and renal outcomes through 2 years, allowing comparisons by sex. Our analysis was able to adjust for many relevant covariates, lending confidence that observed sex differences are not solely due to measured confounders. However, there are also limitations. This is a secondary observational analysis, so causality cannot be established. Residual confounding by unmeasured variables (e.g., differential exposure to nephrotoxins or ventilation practices by sex) remains possible. Our definition of AKI relied on serum creatinine, which is imperfect in neonates, and misclassification bias may have occurred. Furthermore, we did not evaluate the timing of AKI event. Given this, we were unable to fully capture the temporality of relationship between AKI and other complications (for example, PDA). This limitation may have resulted in misclassification of early vs. late exposures. We also note that a recent analysis in this cohort reported no sex differences in AKI incidence [17]. However, differences between studies include variations in inclusion criteria, the timing and frequency of creatinine values included, and the analytic handling of missing data. Our analysis applied a stricter KDIGO-based definition with broader creatinine capture and included more infants in the PENUT cohort based on the analytical questions evaluated, which may increase sensitivity of mild AKI diagnosis. These methodologic differences may account for our divergent findings. Additionally, blood pressure measurements in toddlers can be variable and influenced by multiple environmental factors; yet, the high rates we observed likely indicate real trends. The cohort consists of extremely preterm infants enrolled in a clinical trial, so findings may not generalize to less preterm infants or different settings. Finally, while we identified associations with male sex, the biologic underpinnings require further elucidation beyond the scope of this study.
In summary, we report that male sex is associated with worse short and long-term renal outcomes in extremely preterm infants. These findings reinforce the concept that sex is an important biologic variable in neonates, and emphasizes the need to incorporate sex-disaggregated analyses in neonatal trials and observational studies [29]. Future work should focus on the mechanisms driving these disparities — for example, sex-specific differences in kidney development, hormonal influences, or immunologic responses to kidney injury. Given the modest effect size observed, our findings do not support altering existing monitoring practices based solely on biologic sex. However, sex may serve as one of multiple factors informing broader risk assessment in premature neonates. By acknowledging and investigating sex differences, we can better tailor care and improve long-term outcomes for these vulnerable children.
Supplementary Information
Below is the link to the electronic supplementary material.
Acknowledgements
We would like to thank Lynn Dill, RN, and Emily Pao for their assistance in coordinating the REPaIReD study. We would like to thank the investigators, clinicians, research personnel, study teams and families who participated in the PENUT Trial.
Author contribution
Drs. Starr, Griffin and Soranno had full access to all of the data used in the study and take responsibility for the integrity and accuracy of the report.Concept and design: Drs. Starr and Soranno. Acquisition, analysis or interpretation of data: Drs. Starr, Griffin, and Askenazi. Drafting of the manuscript: Dr. Starr. Critical revision of the manuscript for important intellectual content: Drs. Starr, Griffin, Steflik, Lingappan, Gillen, Selewski, Askenazi, Menon, Slagle, and Soranno. Obtained funding: Drs. Starr and Askenazi. Administrative, technical, or material support: Drs. Starr and Askenazi. Supervision: Drs. Selewski, Askenazi and Soranno.
Funding
This study is part of the ALMOND study that is being coordinated by the Neonatal Kidney Collaborative. Statistical support for this study is provided by Nuwellis. The original study, the Preterm Erythropoietin Neuroprotection Trial (PENUT trial) was funded by the NINDS (U01 NS077953, U01 NS077955). Kidney-specific data collection was funded by the NIDDK as part of the Recombinant Erythropoietin for Protection of Infant Renal Disease (REPaIReD) Study (R01 DK103608). This work was supported in part by R01 DK103608 (DA) and by K23HL168362 (MS). The funding sources for this study had no role in the design and conduct of the study; collection, management, analysis, and interpretation of the data; preparation, review, or approval of the manuscript; and decision to submit the manuscript for publication. Other funding which was not used to support this manuscript includes: R01HL144775 and R01HL168066 (KL).
Data availability
De‐identified individual participant data are available through the NINDS Data Archive: https://www.ninds.nih.gov/Current-Research/Research-Funded-NINDS/Clinical-Research/Archived-Clinical-Research-Datasets. The data includes de‐identified data with data dictionaries, in addition to study protocol, the statistical analysis plan, and the informed consent form.
Declarations
Conflict of interest
All authors report no real or perceived conflicts of interest that could affect the study design, collection, analysis, and interpretation of data, the writing of the report, or the decision to submit the manuscript for publication. For full disclosure, we provide the additional list of authors’ other commitments and funding sources that are not directly related to this study. Dr. Askenazi consults for Nuwellis and Seastar. He is Chief Scientific Officer of Zorro-Flow Inc. He has invention rights for urine collection and fluid balance systems.
Prior presentation of study data
Presented as a poster at the 2024 International Neonatal Nephrology Symposium.
Footnotes
Publisher's Note
Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
References
- 1.O’Driscoll DN, McGovern M, Greene CM, Molloy EJ (2018) Gender disparities in preterm neonatal outcomes. Acta Paediatr 107:1494–1499 [DOI] [PubMed] [Google Scholar]
- 2.van Westering-Kroon E, Huizing MJ, Villamor-Martínez E, Villamor E (2021) Male disadvantage in oxidative stress-associated complications of prematurity: a systematic review, meta-analysis and meta-regression. Antioxidants 10:1490 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 3.Vu HD, Dickinson C, Kandasamy Y (2018) Sex difference in mortality for premature and low birth weight neonates: a systematic review. Am J Perinatol 35:707–715 [DOI] [PubMed] [Google Scholar]
- 4.Laughon MM, Langer JC, Bose CL, Smith PB, Ambalavanan N, Kennedy KA, Stoll BJ, Buchter S, Laptook AR, Ehrenkranz RA, Cotten CM, Wilson-Costello DE, Shankaran S, Van Meurs KP, Davis AS, Gantz MG, Finer NN, Yoder BA, Faix RG, Carlo WA, Schibler KR, Newman NS, Rich W, Das A, Higgins RD, Walsh MC (2011) Prediction of bronchopulmonary dysplasia by postnatal age in extremely premature infants. Am J Respir Crit Care Med 183:1715–1722 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 5.Kent AL, Wright IM, Abdel-Latif ME (2012) Mortality and adverse neurologic outcomes are greater in preterm male infants. Pediatrics 129:124–131 [DOI] [PubMed] [Google Scholar]
- 6.Bairey Merz CN, Dember LM, Ingelfinger JR, Vinson A, Neugarten J, Sandberg KL, Sullivan JC, Maric-Bilkan C, Rankin TL, Kimmel PL, Star RA, Participants of the National Institute of Diabetes and Digestive and Kidney Diseases Workshop on “sex and the kidneys” (2019) Sex and the kidneys: current understanding and research opportunities. Nat Rev Nephrol 15:776–783 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 7.Neugarten J, Golestaneh L (2019) Influence of sex on the progression of chronic kidney disease. Mayo Clin Proc 94:1339–1356 [DOI] [PubMed] [Google Scholar]
- 8.Neugarten J, Golestaneh L (2018) Female sex reduces the risk of hospital-associated acute kidney injury: a meta-analysis. BMC Nephrol 19:314 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 9.Neugarten J, Golestaneh L (2016) The effect of gender on aminoglycoside-associated nephrotoxicity. Clin Nephrol 86:183–189 [DOI] [PubMed] [Google Scholar]
- 10.Neugarten J, Golestaneh L (2013) Gender and the prevalence and progression of renal disease. Adv Chronic Kidney Dis 20:390–395 [DOI] [PubMed] [Google Scholar]
- 11.Hammond JD 2nd, Kielt MJ, Conroy S, Lingappan K, Austin ED, Eldredge LC, Truog WE, Abman SH, Nelin LD, Guaman MC (2023) Exploring the association of male sex with adverse outcomes in severe bronchopulmonary dysplasia: a retrospective, multicenter cohort study. Chest 165:610–620 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 12.Soranno DE, Awdishu L, Bagshaw SM, Basile D, Bell S, Bihorac A, Bonventre J, Brendolan A, Claure-Del Granado R, Collister D, Curtis LM, Dolan K, Fuhrman DY, Habeeb Z, Hutchens MP, Kashani KB, Lumlertgul N, McCulloch M, Menon S, Mohamed A, Pannu N, Reue K, Ronco C, Sahay M, See E, Zappitelli M, Mehta R, Ostermann M (2025) The role of sex and gender in acute kidney injury-consensus statements from the 33rd acute disease quality initiative. Kidney Int 107:606–616 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 13.Askenazi DJ, Heagerty PJ, Schmicker RH, Griffin R, Brophy P, Juul SE, Mayock DE, Goldstein SL, Hingorani S (2020) Prevalence of acute kidney injury (AKI) in extremely low gestational age neonates (ELGAN). Pediatr Nephrol 35:1737–1748 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 14.Askenazi DJ, Heagerty PJ, Schmicker RH, Brophy P, Juul SE, Goldstein SL, Hingorani S (2021) The impact of erythropoietin on short- and long-term kidney-related outcomes in neonates of extremely low gestational age. Results of a multicenter, double-blind, placebo-controlled randomized clinical trial. J Pediatr 232:65-72.e7 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 15.Juul SE, Comstock BA, Wadhawan R, Mayock DE, Courtney SE, Robinson T, Ahmad KA, Bendel-Stenzel E, Baserga M, LaGamma EF, Downey LC, Rao R, Fahim N, Lampland A, Frantz Iii ID, Khan JY, Weiss M, Gilmore MM, Ohls RK, Srinivasan N, Perez JE, McKay V, Vu PT, Lowe J, Kuban K, O’Shea TM, Hartman AL, Heagerty PJ (2020) A randomized trial of erythropoietin for neuroprotection in preterm infants. N Engl J Med 382:233–243 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 16.Starr MC, Griffin R, Gist KM, Segar J, Raina R, Guillet R, Nesargi S, Menon S, Anderson N, Askenazi D, Selewski D (2022) The impact of fluid balance on short- and long-term respiratory outcomes in extremely premature neonates: results from the PENUT study. American Society of Pediatric Nephrology Annual Meeting, Denver CO [DOI] [PMC free article] [PubMed] [Google Scholar]
- 17.Turner MJ, Griffin R, Schuh M, Sanderson K, Slagle C, Menon S, Askenazi D, Gist K (2025) Acute kidney injury and subsequent risk of late-onset infection among extremely low gestational age newborns. J Perinatol 45:1450–1455 [DOI] [PubMed] [Google Scholar]
- 18.Askenazi D, Patil NR, Ambalavanan N, Balena-Borneman J, Lozano DJ, Ramani M, Collins M, Griffin RL (2015) Acute kidney injury is associated with bronchopulmonary dysplasia/mortality in premature infants. Pediatr Nephrol 30:1511–1518 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 19.Starr MC, Boohaker L, Eldredge LC, Menon S, Griffin R, Mayock DE, Li L, Askenazi D, Hingorani S (2020) Acute kidney injury and bronchopulmonary dysplasia in premature neonates born less than 32 weeks’ gestation. Am J Perinatol 37:341–348 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 20.Wallace SW, Geers ER, Niehaus JZ, Cristea AI, Starr MC (2024) Kidney complications in children with bronchopulmonary dysplasia. Pediatr Res 97:2431–2435 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 21.von Elm E, Altman DG, Egger M, Pocock SJ, Gøtzsche PC, Vandenbroucke JP (2007) Strengthening the reporting of observational studies in epidemiology (STROBE) statement: guidelines for reporting observational studies. BMJ 335:806–808 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 22.Zappitelli M, Ambalavanan N, Askenazi DJ, Moxey-Mims MM, Kimmel PL, Star RA, Abitbol CL, Brophy PD, Hidalgo G, Hanna M, Morgan CM, Raju TNK, Ray P, Reyes-Bou Z, Roushdi A, Goldstein SL (2017) Developing a neonatal acute kidney injury research definition: a report from the NIDDK neonatal AKI workshop. Pediatr Res 82:569–573 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 23.Jensen EA, Dysart K, Gantz MG, McDonald S, Bamat NA, Keszler M, Kirpalani H, Laughon MM, Poindexter BB, Duncan AF, Yoder BA, Eichenwald EC, DeMauro SB (2019) The diagnosis of bronchopulmonary dysplasia in very preterm infants. An evidence-based approach. Am J Respir Crit Care Med 200:751–759 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 24.Jensen EA, Wright CJ (2018) Bronchopulmonary dysplasia: the ongoing search for one definition to rule them all. J Pediatr 197:8–10 [DOI] [PubMed] [Google Scholar]
- 25.Hingorani S, Schmicker R, Ahmad KA, Frantz ID, Mayock DE, La Gamma EF, Baserga M, Khan JY, Gilmore MM, Robinson T, Brophy P, Heagerty PJ, Juul SE, Goldstein S, Askenazi D (2022) Prevalence and risk factors for kidney disease and elevated BP in 2-year-old children born extremely premature. Clin J Am Soc Nephrol 17:1129–1138 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 26.Boghossian NS, Geraci M, Edwards EM, Horbar JD (2018) Sex differences in mortality and morbidity of infants born at less than 30 weeks’ gestation. Pediatrics 142:e20182352 [DOI] [PubMed] [Google Scholar]
- 27.Ejiawoko A, Lee HC, Lu T, Lagatta J (2019) Home oxygen use for preterm infants with bronchopulmonary dysplasia in California. J Pediatr 210:55-62.e51 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 28.Menendez-Castro C, Cordasic N, Fahlbusch FB, Woelfle J, Hilgers KF, Hartner A (2023) Sex differences in long-term kidney fibrosis following neonatal nephron loss during ongoing nephrogenesis. Mol Cell Pediatr 10:8 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 29.Lingappan K, Alur P, Eichenwald E (2023) The need to address sex as a biological variable in neonatal clinical studies. J Pediatr 255:17–21 [DOI] [PMC free article] [PubMed] [Google Scholar]
- 30.Flynn JT, Kaelber DC, Baker-Smith CM, Blowey D, Carroll AE, Daniels SR, de Ferranti SD, Dionne JM, Falkner B, Flinn SK, Gidding SS, Goodwin C, Leu MG, Powers ME, Rea C, Samuels J, Simasek M, Thaker VV, Urbina EM (2017) Clinical practice guideline for screening and management of high blood pressure in children and adolescents. Pediatrics 140(3):e20171904. 10.1542/peds.2017-1904 [DOI] [PubMed]
Associated Data
This section collects any data citations, data availability statements, or supplementary materials included in this article.
Supplementary Materials
Data Availability Statement
De‐identified individual participant data are available through the NINDS Data Archive: https://www.ninds.nih.gov/Current-Research/Research-Funded-NINDS/Clinical-Research/Archived-Clinical-Research-Datasets. The data includes de‐identified data with data dictionaries, in addition to study protocol, the statistical analysis plan, and the informed consent form.