Leptospirosis diagnosis — DTM&H MCQ

Correct answer: C — Microscopic agglutination test for Leptospira

The correct answer is C, Microscopic agglutination test for Leptospira. The clinical picture of fever, headache, right hypochondrial pain, jaundice, and bilateral subconjunctival haemorrhage (conjunctival suffusion) after flood exposure in a farmer in Southeast Asia is classic for Weil's disease, the icteric form of leptospirosis, acquired through contact with water contaminated by rodent or livestock urine. The microscopic agglutination test (MAT) uses live Leptospira serovar panels to detect and titrate specific agglutinating antibodies, and remains the internationally recognised reference (gold standard) serological test used to confirm the diagnosis and identify the infecting serogroup. Flood-associated outbreaks of leptospirosis are well documented in tropical settings, and conjunctival suffusion/haemorrhage combined with jaundice strongly points away from the other febrile illnesses listed. Why the other options are wrong: E. Blood cultures for S. typhi: Typhoid causes prolonged fever with relative bradycardia and abdominal symptoms but does not typically produce conjunctival haemorrhage or the acute hepatorenal picture described; blood culture also has poor sensitivity by day 3 of illness in leptospirosis and is not the confirmatory test for this presentation. D. Hepatitis B surface antigen: Viral hepatitis causes jaundice but not the acute febrile onset with headache, myalgia and subconjunctival haemorrhage, and HBsAg reflects hepatitis B infection, not leptospirosis. A. Dengue NS1 antigen: Dengue can cause fever and haemorrhagic signs but typically presents with thrombocytopenia, rash and retro-orbital pain rather than jaundice and right hypochondrial pain from hepatic/renal involvement; NS1 detects dengue antigen only, not Leptospira. B. Widal test: The Widal test detects antibodies to Salmonella typhi and has poor sensitivity and specificity, and is unrelated to the leptospiral aetiology suggested by this history. Key point: Post-flood fever with jaundice and conjunctival suffusion/haemorrhage is leptospirosis until proven otherwise, and MAT is the confirmatory serological gold standard.

Reference: UK Health Security Agency, Leptospirosis: guidance and diagnosis, GOV.UK (2023): historically leptospirosis confirmatory testing was performed using the microscopic agglutination test (MAT) as the reference serological method. https://www.gov.uk/guidance/leptospirosis