What are the laboratory presentations of leptospirosis?

Laboratory Presentation of Leptospirosis

The laboratory presentation of leptospirosis includes leukocytosis with polymorphonuclear predominance, proteinuria and hematuria on urinalysis, elevated bilirubin with disproportionately mild transaminase elevation, and alterations in renal function tests, with anemia developing if significant hemorrhage occurs. 1

Hematologic Findings

• Leukocytosis with polymorphonuclear cell predominance is a characteristic finding in leptospirosis 1
• Anemia can occur if there is significant hemorrhage, particularly in severe forms of the disease 1

Urinalysis Abnormalities

• Proteinuria is commonly found on urine analysis 1
• Hematuria is commonly found on urine analysis 1
• These urinary findings reflect the renal involvement that characterizes leptospirosis, particularly in severe cases 1

Hepatic Function Tests

• Elevated bilirubin with mild elevation of transaminases is a distinctive pattern in leptospirosis 1
• This pattern of disproportionate hyperbilirubinemia relative to transaminase elevation helps distinguish leptospirosis from viral hepatitis, which typically shows more pronounced transaminase elevations 1
• This is particularly important as leptospirosis may be misdiagnosed as viral hepatitis in patients presenting with fever and jaundice 1

Renal Function Tests

• Alterations in renal function tests are commonly found, reflecting the hepato-renal involvement characteristic of severe leptospirosis (Weil's disease) 1
• Renal dysfunction is a hallmark of severe disease and requires close monitoring 2

Diagnostic Confirmation Methods

Serology (Most Common Method)

• Serology is the most common method for confirming the diagnosis of leptospirosis 1
• IgM titers >1:320 are suggestive of leptospirosis 1
• IgM titers of 1:80 to 1:160 are consistent with early infection 1
• Convalescent serology (>10 days after symptom onset) can confirm the diagnosis 1
• IgM typically appears 6-10 days after symptom onset, which is a limitation during the acute phase 2, 3
• The microscopic agglutination test (MAT) on paired samples remains the serologic reference standard, though its accuracy is limited with pooled sensitivity of 68% and specificity of 75% 4

Blood Cultures

• Blood cultures can confirm the diagnosis, especially if taken in the first 5 days before antibiotics 1
• Isolation of leptospires can take up to months and does not contribute to early diagnosis 3

Molecular Testing

• PCR targeting the Lfb1 gene can confirm diagnosis in the early phase prior to detectable antibody titers 3, 4
• Nucleic acid amplification testing (NAAT) typically requires 1-2 hours turnaround time 5

Clinical Pitfalls

• Direct observation by darkfield microscopy is unreliable and not recommended 3
• The limitation of serology is that antibodies are lacking at the acute phase of disease, necessitating clinical suspicion and empiric treatment 3
• Laboratory diagnostic tests vary depending on disease stage, requiring combination of diagnostic tests using appropriate specimens at each stage 6