Clinical Phenotype and Outcomes of Multisystem Inflammatory Syndrome in Children According to SARS‐CoV‐2 Laboratory Status During 2020–2021: A Retrospective Observational Study

ABSTRACT

Background and Aims

Multisystem inflammatory syndrome in children (MIS‐C) is a post‐infectious inflammatory condition associated with severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2) that can affect multiple organ systems. Data comparing clinical features and outcomes between MIS‐C patients with and without laboratory evidence of coronavirus disease 2019 (COVID‐19) remain limited.

Methods

This retrospective observational study was conducted at Mofid Children's Hospital, a pediatric referral center in Tehran, Iran, between March 2020 and July 2021. Children diagnosed with MIS‐C based on World Health Organization (WHO) criteria were included and classified into groups with and without laboratory evidence of SARS‐CoV‐2 infection according to PCR and/or IgG serology results. Demographic data, clinical manifestations, laboratory and imaging findings, treatments, and outcomes were compared between the groups using univariate and multivariable analyses. A p‐value < 0.05 was considered statistically significant.

Results

A total of 121 patients with MIS‐C were included, of whom 44 (36.4%) had laboratory evidence of SARS‐CoV‐2 infection. The mean age was 4.74 ± 3.99 years, and 61.2% were male. ICU admission occurred in 21.5% and mortality was 2.5%. In univariate analysis, fever during the first 24 h of hospitalization and antiviral use were more frequent in the group with laboratory evidence, whereas conjunctivitis and elevated C‐reactive protein levels were more common among the group without laboratory evidence; however, CRP did not differ significantly between groups when analyzed as a continuous variable. In multivariable analysis, fever during the first 24 h of hospitalization was the only variable associated with laboratory evidence of SARS‐CoV‐2 infection (aOR 2.57; 95% CI 1.05–6.30; p = 0.040).

Conclusion

MIS‐C presents with broad and largely overlapping clinical, laboratory, and imaging features regardless of SARS‐CoV‐2 laboratory status, and the few observed differences were exploratory and of limited discriminative value. These findings highlight the importance of considering MIS‐C even in the absence of laboratory confirmation of SARS‐CoV‐2 infection.

1. Introduction

The coronavirus disease 2019 (COVID‐19) outbreak, caused by severe acute respiratory syndrome coronavirus 2 (SARS‐CoV‐2), originated in Wuhan, China, in late 2019 and rapidly spread worldwide, causing significant morbidity and mortality [1]. Although children were initially thought to be relatively spared compared to adults, it soon became clear that pediatric populations could also get infected, typically presenting with mild or asymptomatic disease [2, 3]. Over time, a severe post‐infectious hyperinflammatory condition known as multisystem inflammatory syndrome in children (MIS‐C) was recognized in association with SARS‐CoV‐2 infection [4].

MIS‐C is believed to result from an exaggerated immune response usually occurring 2–6 weeks after the initial infection. This dysregulated immune activation is described as a cytokine storm, leading to widespread inflammation, endothelial dysfunction, and multiorgan involvement rather than direct viral injury [5, 6]. Commonly affected organs include the gastrointestinal, mucocutaneous, cardiac, hematologic, renal, and respiratory systems [7].

Laboratory findings reveal a hyperinflammatory state with elevated C‐reactive protein (CRP), erythrocyte sedimentation rate (ESR), procalcitonin, ferritin, and d‐dimer levels, along with abnormalities in blood cell counts. Cardiac involvement is a prominent feature of MIS‐C, characterized by elevated troponin and N‐terminal pro‐B‐type natriuretic peptide (pro‐BNP) as well as echocardiographic changes such as left ventricular systolic dysfunction, pericardial effusion, and coronary artery disorders [8, 9, 10]. In contrast, pulmonary imaging findings are less frequent and generally nonspecific, including peribranchial thickening, bilateral ground‐glass opacities, pulmonary consolidations, and pleural effusions [11].

Evidence of prior SARS‐CoV‐2 infection is not always detectable at the time of MIS‐C presentation, as 30%–50% of children are reverse transcription polymerase chain reaction (RT‐PCR) positive, more than 80% demonstrate positive serologic tests, and 60% report a history of close contact with a COVID‐19 case [7, 12]. Because the case definition is nonspecific and confirmatory laboratory testing does not exist, it might be difficult to distinguish MIS‐C from other conditions with overlapping clinical manifestations such as severe acute COVID‐19 and Kawasaki disease. Therefore, MIS‐C should be considered as a differential diagnosis in critically ill patients even with negative polymerase chain reaction (PCR) and serologic test results [13].

Management of MIS‐C requires a multidisciplinary approach. First‐line therapy generally consists of intravenous immunoglobulin (IVIG) and corticosteroids, while antiplatelet and anticoagulant agents are used when clinically indicated. Since MIS‐C typically appears in the post‐infectious phase and is primarily immune‐mediated, antiviral therapy is not routinely recommended, except in selected cases where acute SARS‐CoV‐2 infection cannot be confidently excluded [14, 15].

Despite growing evidence, data directly comparing clinical manifestations, paraclinical findings, and outcomes between MIS‐C patients with and without laboratory evidence of SARS‐CoV‐2 infection remain limited; therefore, the present study aimed to evaluate these parameters in pediatric patients diagnosed with MIS‐C.

2. Methods

2.1. Study Design

This retrospective observational study was conducted at Mofid Children's Hospital, Tehran, Iran, between March 2020 and July 2021. Of 189 hospitalized children with a final diagnosis of MIS‐C, 68 were excluded because they did not fully meet the latest WHO criteria or had duplicate records, leaving 121 patients for analysis [16].

A documented history of fever for > 3 days was mandatory, as required by WHO. Fever reported elsewhere in this manuscript as a “vital sign” (e.g., Table 3) refers to a measurement at a specific time point (at admission, or during the first 24 h of hospitalization), which may be absent due to fluctuation of fever or antipyretic use. The complete WHO case definition and its operationalization are presented in Table 1A.

Table 3.

Vital sign assessment at admission and fever during 24 h of hospitalization among pediatric patients with MIS‐C according to laboratory evidence of SARS‐CoV‐2 infection.

Variable	Without laboratory evidence MIS‐C patients n/N (%)	With laboratory evidence MIS‐C patients n/N (%)	Total n/N (%)	p value
Fever	29/77 (37.7%)	22/44 (50%)	51/121 (42.1%)	0.19
Fever during first 24 h	42/77 (54.5%)	34/44 (77.3%)	76/121 (62.8%)	0.01
Tachycardia	28/73 (38.4%)	21/43 (48.8%)	49/116 (42.2%)	0.52
Hypertension	31/69 (44.9%)	18/43 (41.9%)	49/112 (43.8%)	0.96
Hypotension	6/69 (8.7%)	4/43 (9.3%)	10/112 (8.9%)	0.96
Tachypnea	17/70 (24.3%)	10/43 (23.3%)	27/113 (23.9%)	0.90
Hypoxemia	9/57 (15.8%)	3/39 (7.7%)	12/96 (12.5%)	0.35

Note: Tachycardia, tachypnea, and hypotension were defined according to age‐specific thresholds as follows: heart rate > 160 beats/min, respiratory rate > 60 breaths/min, and SBP < 60 mmHg for neonates (< 28 days); heart rate > 140 beats/min, respiratory rate > 40 breaths/min, and SBP < 70 mmHg for infants aged 1–12 months; heart rate > 130 beats/min, respiratory rate > 40 breaths/min, and SBP < 70 + (2 × age in years) mmHg for children aged 1–3 years; heart rate > 120 beats/min, respiratory rate > 40 breaths/min, and SBP < 70 + (2 × age in years) mmHg for children aged 3–5 years; heart rate > 100 beats/min, respiratory rate > 30 breaths/min, and SBP < 70 + (2 × age in years) mmHg for children aged 5–10 years; and heart rate > 100 beats/min, respiratory rate > 30 breaths/min, and SBP < 90 mmHg for children older than 10 years. Fever was defined as a forehead temperature >38°C measured using a digital infrared thermometer, and hypoxemia was defined as SpO₂ < 92%. Hypertension was defined as blood pressure ≥ 95th percentile for age, sex, and height. The age‐specific thresholds were based on established pediatric clinical reference ranges.

Abbreviations: SBP, systolic blood pressure; SpO₂, peripheral oxygen saturation.

Table 1A.

WHO case definition of MIS‐C (May 2020) and its operationalization in this study.

Criterion	WHO definition	Operationalization in this study
Age	Children and adolescents 0–19 years	Hospitalized children ≤ 19 years
Fever	Fever > 3 days	Documented history of fever > 3 days
Clinical features (at least two)	(a) Rash, bilateral non‐purulent conjunctivitis, or mucocutaneous inflammation signs (oral, hands, or feet); (b) hypotension or shock; (c) features of myocardial dysfunction, pericarditis, valvulitis, or coronary abnormalities (including echocardiographic findings or elevated troponin/NT‐proBNP); (d) evidence of coagulopathy (prolonged PT or PTT, elevated d‐dimer); (e) acute gastrointestinal problems (diarrhea, vomiting, or abdominal pain)	Mucocutaneous and gastrointestinal symptoms as recorded in the medical files. Hypotension: age‐specific thresholds (Table 3 footnote). Cardiac: echocardiography/troponin as defined in Methods. Coagulopathy: PT > 13.5 s, PTT > 35 s, or d‐dimer > 200 ng/mL (Table 4 footnote).
Inflammation	Elevated markers of inflammation (e.g., ESR, CRP, or procalcitonin)	ESR > 20 mm/h and/or CRP > 10 mg/L (institutional reference ranges)
Exclusion of other causes	No other obvious microbial cause of inflammation, including bacterial sepsis and staphylococcal or streptococcal shock syndromes	Patients with an alternative explanation for the inflammatory syndrome documented in the medical file (e.g., confirmed bacterial sepsis, staphylococcal or streptococcal toxic shock syndrome, or another established infection) were excluded.
SARS‐CoV‐2 evidence	Evidence of COVID‐19 (RT‐PCR, antigen test, or serology positive) or contact with a COVID‐19 patient	Not mandatory in this study: children with negative or untested SARS‐CoV‐2 results and no documented contact were included if all other criteria were met (a deviation from the WHO definition)

Abbreviations: CRP, C‐reactive protein; ESR, erythrocyte sedimentation rate; NT‐proBNP, N‐terminal pro‐B‐type natriuretic peptide; PT, prothrombin time; PTT, partial thromboplastin time.

Since this study was conducted during the early phase of the COVID‐19 pandemic, potential exposure to SARS‐CoV‐2 was considered plausible even in the absence of documented contact. Therefore, children with negative or untested SARS‐CoV‐2 results and no documented history of close contact were still considered eligible if all other criteria were met.

SARS‐CoV‐2 PCR was performed in all patients within the first 72 h of hospitalization. Serology was limited to immunoglobulin G (IgG) antibodies, as MIS‐C is a post‐infectious condition [17]; however, testing was performed in only 34 of 121 patients (28.1%) because of limited resources. Because selection was not systematic, serology results cannot be generalized to the whole cohort.

2.2. Patient Classification

Patients with a positive PCR and/or IgG were classified as having laboratory evidence of SARS‐CoV‐2 infection. All others were classified as without laboratory evidence; this group includes patients with a negative PCR who had negative IgG or, in most cases, no serologic testing, and therefore does not necessarily represent uninfected children. PCR‐positive, serology‐positive, either‐positive, neither‐positive, and untested subgroups are reported in Table 1.

Table 1.

Classification of 121 MIS‐C patients by SARS‐CoV‐2 PCR and IgG serology results.

	IgG positive	IgG negative	IgG not tested	Total
PCR positive	3	0	26	29 (24.0%)
PCR negative	15	16	61	92 (76.0%)
Total	18	16	87	121

2.3. Data Collection

Data were extracted from medical files using a questionnaire based on the WHO COVID‐19 Clinical Case Report Form [18]. Collected variables included demographics, clinical features, vital signs, laboratory findings, imaging, cardiac evaluations, treatments, and outcomes. Laboratory tests were recorded within the first 24 h of admission, except troponin (any time during hospitalization). Treatments, echocardiography, and imaging were considered throughout hospitalization.

Cardiac involvement was defined as at least one clinically significant abnormality: left ventricular systolic dysfunction (EF < 55%), LV hypertrophy, coronary artery dilation or aneurysm, moderate or larger pericardial effusion, pericarditis, valvulitis/endocarditis, myocarditis/cardiomyopathy, or elevated troponin. Minor findings (e.g., trivial or small pericardial effusion) were excluded. Echocardiograms were performed and interpreted by a pediatric cardiologist. NT‐proBNP was not used because it was not measured in most patients.

2.4. Statistical Analysis

Analyses were performed with SPSS version 27. Continuous variables were summarized as mean ± SD, or median (IQR) when non‐normally distributed (e.g., CRP), and categorical variables as frequencies and percentages. Groups were compared using the chi‐square or Fisher's exact test, the independent‐samples t‐test, or the Mann–Whitney U test, as appropriate. No correction for multiple testing was applied; findings with p‐values between 0.03 and 0.05 were therefore considered exploratory.

A multivariable logistic regression model was fitted to explore characteristics associated with laboratory evidence of SARS‐CoV‐2 infection. Candidate variables (p < 0.20 in univariable analysis) were restricted to a few clinically relevant characteristics (fever during the first 24 h of hospitalization, cough, conjunctivitis, and age) because only 44 patients had laboratory evidence, keeping about 10 events per variable to limit overfitting [19]. Results are reported as odds ratios with 95% confidence intervals and describe associations, not prediction or causation. All tests were two‐sided, with p < 0.05 considered significant.

3. Results

3.1. Demographic and Epidemiological Characteristics

All 121 patients underwent SARS‐CoV‐2 PCR testing, and 29 (24.0%) were PCR positive. IgG serology was performed in 34 patients (28.1% of the cohort), of whom 18 (52.9% of those tested) were IgG positive; three patients were positive on both PCR and IgG. Overall, 44 patients (36.4%) had laboratory evidence of SARS‐CoV‐2 infection by PCR and/or IgG, whereas 77 (63.6%) had no laboratory evidence of infection. Of these 77 patients, 16 (20.8%) had negative IgG serology, and 61 (79.2%) had not undergone serologic testing (Table 1).

Underlying diseases were observed in 16.5% of cases‐most commonly neurologic, renal, and cardiac conditions‐with no significant difference between the groups (Table 2).

Table 2.

Demographic and epidemiological characteristics of pediatric MIS‐C patients according to laboratory evidence of SARS‐CoV‐2 infection.

Variable	Without laboratory evidence MIS‐C patients N = 77	With laboratory evidence MIS‐C patients N = 44	Total N = 121	p value
Age (yr), mean ± SD	4.36 ± 3.71	5.42 ± 4.41	4.74 ± 3.99	0.16
Male, n (%)	45 (58.4%)	29 (65.9%)	74 (61.2%)	0.42
Underlying diseases, n (%)	16 (20.8%)	4 (9.1%)	20 (16.5%)	0.10
Obesity, n (%)	9 (17.0%)	9 (31.0%)	18 (22.0%)	0.14
Close contact, n (%)	18 (40%)	18 (60%)	36 (48.0%)	0.09

Note: Obesity data were available for 82 patients only (53 without and 29 with laboratory evidence). Percentages were calculated based on available data. Close contact history was available for 75 patients (45 without and 30 with laboratory evidence). Percentages were calculated based on available data^.

3.2. Clinical Presentation

3.2.1. Vital Sign Assessment

All 121 patients had a history of fever prior to admission, in accordance with WHO criteria. However, only 51 patients (42.1%) were febrile at admission, with no significant difference between groups. During the first 24 h of hospitalization, fever was significantly more common in patients with laboratory evidence (77.3% vs. 54.5%; p = 0.01), while other vital sign abnormalities showed no significant difference (Table 3).

3.2.2. Gastrointestinal Involvement

Overall, 84.3% of patients exhibited gastrointestinal symptoms. Vomiting (68.2% vs. 62.3%; p = 0.52), diarrhea (59.1% vs. 49.4%; p = 0.30), and abdominal pain (40.9% vs. 24.7%; p = 0.06) were all more frequent in the group with laboratory evidence but did not reach statistical significance.

3.2.3. Mucocutaneous Involvement

Mucocutaneous manifestations were present in 77.7% of patients, most commonly rash (66.9%), conjunctivitis (43.8%), edema (13.2%), mucosal changes like erythema or strawberry tongue (11.6%), and lymphadenopathy (9.9%). Except for conjunctivitis, which was more frequent in the group without laboratory evidence (56.8% vs. 36.4%; p = 0.03; exploratory finding), other mucocutaneous findings did not differ significantly between the two groups.

3.2.4. Neurologic Involvement

Neurologic symptoms were observed in 23.1% of patients, including headache (11.7%), altered mental status (7.4%), and seizure (6.6%), with no significant intergroup differences.

3.2.5. Respiratory Manifestations

Upper and lower respiratory manifestations were generally uncommon and did not differ significantly between the groups, while cough tended to be more frequent in patients with laboratory evidence (38.6% vs. 23.4%; p = 0.08).

3.3. Paraclinical Findings

3.3.1. Laboratory Abnormalities

The most frequent complete blood count abnormalities were neutrophilia, anemia, and lymphocytopenia, while elevated fibrinogen, ESR, and CRP were the most common inflammatory disturbances. The proportion of patients with elevated CRP was higher in the group without laboratory evidence (86.1% vs. 70.7%; p = 0.04; exploratory finding). However, when CRP was compared as a continuous variable, the median did not differ significantly (30 mg/L [IQR, 25] vs. 20 mg/L [IQR, 40.5]; Mann–Whitney U = 1356.5; p = 0.472). No other laboratory marker differed significantly between groups (Table 4).

Table 4.

Laboratory abnormalities in pediatric patients with MIS‐C according to SARS‐CoV‐2 laboratory status.

Variable	Without laboratory evidence MIS‐C patientsn/N (%)	With laboratoryevidence MIS‐C patientsn/N (%)	Totaln/N (%)	p value
Anemia	55/75 (73.3%)	33/44 (75%)	88/119 (73.9%)	0.84
Decreased HCT	23/75 (30.7%)	15/44 (34.1%)	38/119 (31.9%)	0.70
Leukopenia	10/75 (13.3%)	7/44 (15.9%)	17/119 (14.3%)	0.70
Leukocytosis	32/75 (42.7%)	21/44 (47.7%)	53/119 (44.5%)	0.71
Neutrophilia	60/75 (80%)	38/44 (86.4%)	98/119 (82.4%)	0.76
Lymphocytopenia	48/75 (64%)	35/44 (79.5%)	83/119 (69.7%)	0.14
Thrombocytopenia	24/75 (32.0%)	15/44 (34.1%)	39/119 (33.1%)	0.52
Thrombocytosis	7/75 (9.5%)	7/44 (15.9%)	14/119 (11.9%)	0.52
Increased ESR	62/72 (86.1%)	38/41 (92.7%)	100/113 (88.5%)	0.37
Increased CRP	62/72 (86.1%)	29/41 (70.7%)	91/113 (80.5%)	0.04
Increased fibrinogen	44/46 (95.7%)	33/33 (100%)	77/79 (97.5%)	0.23
Increased Ferritin	8/54 (14.8%)	3/31 (9.7%)	11/85 (12.9%)	0.50
Increased d‐dimer	17/32 (53.1%)	14/21 (66.7%)	31/53 (58.5%)	0.33
Prolonged PT	5/37 (13.5%)	5/29 (17.2%)	10/66 (15.2%)	0.68
Prolonged PTT	15/38 (39.5%)	11/29 (37.9%)	26/67 (38.8%)	0.90
Increased INR	9/37 (24.3%)	11/29 (37.9%)	20/66 (30.3%)	0.23

Note: Percentages were calculated based on available laboratory data for each parameter. Laboratory abnormalities were defined according to institutional reference ranges as follows: hemoglobin < 11 g/dL, hematocrit < 30%, thrombocytopenia < 150 × 10⁹/L, thrombocytosis > 450 × 10⁹/L, PT > 13.5 s, PTT > 35 s, INR > 1.1, ESR > 20 mm/h, CRP > 10 mg/L, ferritin > 800 ng/mL, and fibrinogen > 400 mg/dL, and d‐dimer > 200 ng/mL. Age‐specific reference ranges were applied for leukocyte count and neutrophil counts: leukopenia was defined as WBC < 9000/µL in infants < 1 month and < 5000/µL in older children; leukocytosis as WBC > 30,000/µL in infants < 1 month and > 10,000/µL in older children; neutropenia as neutrophil count < 40%, < 15%, < 32%, and < 32% for age groups < 1 month, 1–60 months, 5–10 years, and > 10 years, respectively; neutrophilia as neutrophil count > 60%, > 35%, > 60%, and > 60%, respectively.

Abbreviations: CRP, C‐reactive protein; ESR, erythrocyte sedimentation rate; INR, international normalized ratio; PT, prothrombin time; PTT, partial thromboplastin time; WBC, white blood cell count.

3.3.2. Renal Function Tests

Renal involvement occurred in 36 of 93 cases (38.7%), including elevated creatinine (50.8%), proteinuria (44.1%), hematuria (30.1%), and increased blood urea nitrogen (BUN) (11.9%). When comparing the groups, no significant differences were observed.

3.3.3. Cardiac Biomarkers and Echocardiography

Troponin was measured in 42 of 121 patients (34.7%), and 35.7% of them had elevated levels (60% of these in the laboratory evidence group, p = 0.09). Echocardiography was performed in 113 patients (93.4%). Only clinically significant findings, as defined in the Methods, were counted as cardiac involvement, and minor abnormalities were not included. On this basis, 44.2% of patients had cardiac involvement (50% with vs. 41.1% without laboratory evidence; p = 0.36). The most frequent qualifying findings were moderate or larger pericardial effusion (38.1%), reduced ejection fraction (19.5%), left ventricular hypertrophy (7.1%), and coronary artery dilation (6.2%). Overall cardiac involvement, defined by elevated troponin and/or clinically significant echocardiographic findings, tended to be more frequent in patients with laboratory evidence (p = 0.06).

3.3.4. Pulmonary Imaging

Among 97 children who underwent chest imaging, pulmonary involvement was observed in 28.9% overall, including 32.4% of patients with laboratory evidence and 27% of those without (p = 0.58). The most common findings were opacities (20.6%) and consolidations (10.3%).

3.4. Treatment Modalities

Pre‐admission medications included antipyretics (61.2%), antibiotics (38.8%), and corticosteroids (13.2%), with no meaningful differences between the two groups. Regarding in‐hospital treatments, the most frequently administered drugs were corticosteroids (91.7%), antibiotics (72.7%), antiplatelets (68.6%), IVIG (59.5%), anticoagulants (26.4%), and inotropes (22.3%). Antivirals were used in 21.5% overall and were significantly more common among patients with laboratory evidence (34.1% vs. 14.3%; p = 0.01). 25.6% of patients received supplemental oxygen, including simple oxygen mask (62.5%), intubation (21.8%), oxygen hood (6.3%), and reservoir mask (6.3%), with no significant differences between groups.

3.5. Clinical Outcomes

Overall, 26 patients (21.5%) required ICU admission; this proportion was numerically higher in patients with laboratory evidence (25% vs. 19.5%; p = 0.48). Three patients (2.5%) died, with no significant intergroup difference (p > 0.99). The mean duration of hospitalization was also similar between groups with an average of 7.61 ± 5.06 days (p = 0.57).

3.6. Multivariable Logistic Regression

In the multivariable logistic regression including all 121 patients, fever during the first 24 h of hospitalization was the only variable associated with laboratory evidence of SARS‐CoV‐2 infection (aOR 2.57; 95% CI 1.05–6.30; p = 0.040). Cough showed a borderline association (aOR 2.35; 95% CI 1.00–5.53; p = 0.051). Conjunctivitis tended to be less frequent in patients with laboratory evidence (aOR 0.48; 95% CI 0.21–1.10; p = 0.082), and older age showed a non‐significant trend toward laboratory evidence (aOR 1.09 per year; 95% CI 0.99–1.20; p = 0.096).

4. Discussion

Our findings demonstrated that most clinical, paraclinical, treatment, and outcome characteristics did not differ significantly according to laboratory evidence of SARS‐CoV‐2 infection, while a few exploratory differences were observed (fever during the first 24 h, conjunctivitis, the proportion with elevated CRP, and antiviral use).

PCR positivity (24.0%) in our study was similar to previous reports. Among the 34 patients who underwent IgG testing (28.1% of the cohort), 52.9% were seropositive; this proportion applies only to tested patients and cannot be extrapolated to the whole study population. It was lower than the nearly 87% described by Whittaker et al. and Nakra et al [20, 21]. Several factors may explain this difference. Serologic testing may have been performed early in the illness, before antibody responses had fully developed. Finally, differences in age distribution and in the circulating viral variants across pandemic waves may have contributed.

The mean age in our research was lower than that reported in most international MIS‐C studies but was consistent with previous Iranian data [14, 22, 23]. This could be explained by differences in population characteristics and referral patterns across different studies. Male predominance and the frequency of underlying diseases were generally comparable to previous reports [24, 25, 26]. Obesity, as a potential risk factor for more severe disease in MIS‐C, was slightly more frequent in the group with laboratory evidence, although this difference was not statistically significant and may warrant further evaluation [27].

Fever during the first 24 h of hospitalization, but not at admission, was more common in patients with laboratory evidence and was the only variable associated with laboratory evidence in the multivariable analysis (aOR 2.57; 95% CI 1.05–6.30), although the confidence interval was wide and this association should be interpreted cautiously. This finding suggests that repeated temperature assessment may capture the fluctuating course of fever more reliably than a single admission measurement.

Clinical manifestations were consistent with established MIS‐C literature, with gastrointestinal and mucocutaneous involvement being the most common [28, 29]. Neurologic manifestations were observed in 23.1% of our patients, lower than that reported in some larger cohorts [14, 30], possibly due to differences in disease severity or variability in the criteria used to define neurologic involvement across studies, as some reports also included symptoms such as irritability or dizziness as neurologic involvement [31, 32]. Conjunctivitis was more frequent in the group without laboratory evidence, a finding less consistently reported previously; however, after adjustment, the association between conjunctivitis and the absence of laboratory evidence was no longer statistically significant, which supports its exploratory nature.

The laboratory findings in both groups closely resembled those previously reported in MIS‐C, with neutrophilia, anemia, lymphopenia, elevated inflammatory markers, and increased d‐dimer levels among the most commonly observed abnormalities [33, 34, 35, 36], and routine laboratory tests did not distinguish children with and without laboratory evidence of SARS‐CoV‐2. The higher proportion of elevated CRP in the group without laboratory evidence was not supported when CRP was analyzed as a continuous variable, suggesting that it is more likely a chance finding, related to the CRP cut‐off used and multiple comparisons, than a true difference, and it should be considered exploratory. Because CRP changes rapidly during illness, the undocumented interval between fever onset and sampling, as well as treatments received before presentation, may also have influenced individual values [12, 37].

Cardiac involvement is one of the most important manifestations of MIS‐C due to its potential impact on disease severity and clinical outcomes. The most frequent echocardiographic abnormalities in our study were pericardial effusion and reduced ejection fraction, findings that are consistent with prior literature [38]. Troponin elevation was also observed at a frequency comparable to that described in earlier studies [39]. Although there was no significant difference in individual cardiac findings between the groups, overall cardiac involvement (troponin elevation and/or echocardiographic changes) was more frequent in the group with laboratory evidence, reaching borderline significance (p = 0.06). This may suggest a tendency toward greater cardiac involvement in patients with laboratory evidence of SARS‐CoV‐2 infection, although larger studies are needed to confirm this observation.

Treatment protocols included corticosteroids (91.7%), antibiotics (72.7%), antiplatelets (68.6%), and IVIG (59.5%). The relatively lower use of IVIG in our study, despite it being considered the predominant therapy, may be related to differences in treatment protocols or limitations in resource availability [25, 35]. Because antivirals are not part of standard MIS‐C management and its prescription was likely influenced by clinicians' pretest suspicion of acute SARS‐CoV‐2 infection, its association with laboratory evidence should not be interpreted as an independent biological characteristic of MIS‐C [40]. However, the use of other treatments was comparable between the two groups.

Importantly, ICU admission, mortality, and hospitalization duration in our study were considerably lower than those reported in a systematic review by Kamfar et al. [10]. This difference could be attributed to earlier recognition of MIS‐C in more recent studies, resulting in faster referral and treatment before progression to severe organ damage [41]. The absence of significant differences in adverse outcomes should be interpreted cautiously, because the study was underpowered for severe outcomes (26 ICU admissions, 3 deaths) and does not establish equivalence between the two groups.

Our study provides real‐world data on MIS‐C during the early phase of the COVID‐19 pandemic, but it has limitations. It was a single‐center, retrospective study conducted in 2020–2021, so the findings may not be generalizable to later variants or the post‐vaccination era. No confirmatory test for MIS‐C exists, and classification by laboratory status was limited: serology (IgG only) was performed in 28.1% of patients, the timing of PCR and serology relative to fever onset could not be standardized, and close‐contact history was incomplete, so some patients may have been misclassified. Some biomarkers (e.g., NT‐proBNP, procalcitonin) were not available for most patients. Finally, many variables were compared without correction for multiple testing, and the regression model was based on only 44 events, so no inference of prediction or causation can be made.

5. Conclusion

Children with and without laboratory evidence of SARS‐CoV‐2 infection had largely similar clinical, laboratory, imaging, treatment, and outcome characteristics, and the few differences observed were exploratory. Because serology was available in only 28.1% of patients and no confirmatory test for MIS‐C exists, the group without laboratory evidence may include children with other inflammatory conditions. Nevertheless, MIS‐C should be considered in children with compatible features and no laboratory evidence of SARS‐CoV‐2 infection, after exclusion of alternative diagnoses.

Author Contributions

Shahnaz Armin: conceptualization, project administration, writing – review and editing, supervision, investigation. Seyede Mahsan Hoseini‐Alfatemi: project administration, writing – review and editing. Roxana Mansour Ghanaiee: conceptualization, validation, supervision. Abdollah Karimi: conceptualization, supervision. Negin Rahimidanesh: writing – original draft, visualization, validation. Arian Ashnaei: writing – original draft, validation, visualization. Amirata Fakhfouri: methodology, writing – original draft. Azadeh Jafrasteh: project administration, data curation. Yasna Soleimani: formal analysis, methodology, writing – original draft, writing – review and editing, software.

Funding

The authors have nothing to report.

Disclosure

All authors have read and approved the final version of the manuscript. Yasna Soleimani had full access to all of the data in this study and takes complete responsibility for the integrity of the data and the accuracy of the data analysis.

Ethics Statement

This study was reviewed and approved by the Ethics Committee of the Research Institute for Children's Health at Shahid Beheshti University of Medical Sciences (ethical code: IR. SBMU. RICH. REC.1402.014). The requirement for informed consent was waived by the Ethics Committee because of the retrospective nature of the study.

Conflicts of Interest

The authors declare no conflicts of interest.

AI Disclosure

ChatGPT (OpenAI) and Claude (Anthropic) were used to assist with grammatical and language editing of the manuscript text; these tools were not used for data analysis, interpretation, or generating tables. All AI‐assisted edits were reviewed carefully and verified by the authors, who take full responsibility for the accuracy and integrity of the final content.

Transparency Statement

Shahnaz Armin affirms that this manuscript is an honest, accurate, and transparent account of the study being reported; that no important aspects of the study have been omitted, and that any discrepancies from the study as planned have been explained.

Data Availability Statement

The data supporting the findings of this study are available from the corresponding author upon request.