A rice-field worker from Kerala presents in September with sudden fever, myalgia, conjunctival suffusion, and jaundice. Urinalysis shows proteinuria and casts. Leptospira IgM ELISA is positive. The microscopic agglutination test (MAT) is the reference serological test. What titer is considered significant in the MAT for diagnosis of acute leptospirosis?
- A MAT titer ≥ 1:20 with a single serum sample
- B MAT titer > 1:5000 only
- C MAT is not used for acute diagnosis; only blood culture is accepted
- D MAT titer ≥ 1:100 in a single serum, or ≥ 1:400 at any time, or 4-fold rise between paired sera ✓
Explanation
The MAT uses live or formalin-killed Leptospira serovars mixed with patient serum; the highest dilution causing ≥ 50% agglutination is the titer. Diagnostic criteria: single titer ≥ 1:100 in a suggestive clinical context (especially in endemic areas), or titer ≥ 1:400 at any time, or a 4-fold or greater rise in titer between acute and convalescent sera drawn 7–14 days apart. The predominant reacting serogroup guides epidemiological understanding, though cross-reactions are common. Blood culture (EMJH medium) is highly specific but takes 6–8 weeks; MAT and IgM ELISA are used for clinical diagnosis.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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