What lab findings support the diagnosis of leptospirosis in a child with suspected infection?

Laboratory Findings Supporting Leptospirosis Diagnosis in Children

In a child with suspected leptospirosis, obtain blood cultures (ideally within first 5 days), complete blood count with differential, comprehensive metabolic panel including liver and renal function tests, urinalysis, and serology (IgM ELISA), as these laboratory findings collectively support the diagnosis and guide management. 1

Hematologic Findings

Complete Blood Count Abnormalities

• Thrombocytopenia (<150,000 cells/mm³) is a key finding, occurring in 65% of pediatric cases and showing significantly lower counts in severe disease with a declining trend as illness progresses. 2, 3
• Leukocytosis (>11,000 cells/mm³) with polymorphonuclear predominance occurs significantly more in severe disease, particularly from day 4-5 of illness. 4, 2
• Progressive decline in hemoglobin (<12.5 gm/dl) from day 1 to day 7 occurs in both mild and severe disease, with more pronounced anemia if significant hemorrhage is present. 4, 2
• An elevated total band count (>1500/mm³) has the highest likelihood ratio (14.5) for detecting bacterial infection in the appropriate clinical context. 5

Biochemical Abnormalities

Hepatic Dysfunction

• Markedly elevated bilirubin (often >580 μmol/L) with disproportionately mild transaminase elevation (AST <200 U/L) is a characteristic pattern that distinguishes leptospirosis from viral hepatitis. 4, 6
• Jaundice occurs in 70% of pediatric cases and indicates severe disease (Weil's disease). 4, 3
• Elevated transaminases occur in 56% of cases but remain relatively mild compared to the degree of hyperbilirubinemia. 3

Renal Dysfunction

• Rapidly progressive acute kidney injury occurs in 79% of pediatric cases and is a hallmark of severe leptospirosis. 3, 6
• Alterations in renal function tests (elevated creatinine and BUN) develop early and progress rapidly. 4

Urinalysis Findings

• Proteinuria and hematuria on urinalysis are important laboratory findings that contribute to diagnosis and are included in Faine's diagnostic criteria. 1, 4

Microbiological Diagnosis

Blood Cultures

• Blood cultures should be obtained before antibiotics if this causes no significant delay (<45 minutes), ideally within the first 5 days of illness when bacteremia is present. 1, 4
• Three or more blood cultures should be taken at least 1 hour apart for optimal yield. 4
• Culture can take weeks to months and does not contribute to early diagnosis but provides definitive confirmation. 7

Serological Testing

• IgM ELISA with titers >1:320 (or >100 U/ml) is diagnostic of leptospirosis. 1, 4
• IgM titers of 1:80 to 1:160 are consistent with early infection and should prompt repeat testing. 4
• Convalescent serology with microscopic agglutination test (MAT) should be repeated >10 days after symptom onset, with a fourfold or greater increase in titer between acute and convalescent specimens confirming diagnosis. 1, 4
• Faine's criteria using MAT ≥1:400 and/or PCR has 95.45% sensitivity but only 56.86% specificity. 1

Molecular Testing

• Nucleic acid amplification testing (NAAT/PCR) can confirm diagnosis in the early phase before antibodies develop, with turnaround time of 1-2 hours. 1, 7
• PCR is particularly valuable in the first week when serology is often negative. 7

Critical Pitfalls to Avoid

• Do not wait for serological confirmation before starting antibiotics, as serology is often negative in the first week and each hour of delay increases mortality. 1, 7
• Do not mistake leptospirosis for viral hepatitis in patients with fever and jaundice—the pattern of markedly elevated bilirubin with mild transaminase elevation is key. 1, 6
• Do not rely on darkfield microscopy for direct observation of leptospires, as it is unreliable and not recommended. 7
• Do not use urine for culture, as it is not suitable for leptospira isolation. 1
• Recognize that afebrile presentations can occur in severe leptospirosis (<1% of cases), so do not exclude the diagnosis based solely on absence of fever in endemic areas with appropriate exposure history. 6

Diagnostic Algorithm

When evaluating a child with suspected leptospirosis based on exposure history (flood water, contaminated fresh water, animal contact within past 2-20 days) and clinical features (biphasic fever, severe myalgias, conjunctival suffusion): 1

1. Immediately obtain: CBC with differential, comprehensive metabolic panel (liver and renal function), urinalysis, blood cultures (if <5 days of symptoms), and IgM ELISA. 1

2. Start antibiotics immediately without waiting for laboratory confirmation if clinical suspicion is high. 1

3. Interpret results in context: Thrombocytopenia + leukocytosis + proteinuria/hematuria + elevated bilirubin disproportionate to transaminases + acute kidney injury = highly suggestive pattern. 2, 3, 6

4. Obtain convalescent serology >10 days after symptom onset to confirm diagnosis retrospectively. 1, 4