A sewage worker presented with abdominal pain, jaundice, conjunctival injection, and blood in the urine for the past 5 days. Which of the following is the investigation of choice?

Correct Answer: A. Microscopic agglutination test

The clinical presentation—abdominal pain, jaundice, conjunctival injection (without exudate), and hematuria in a sewage worker over 5 days—is pathognomonic for leptospirosis, caused by Leptospira interrogans. Occupational exposure to contaminated water/sewage is the key epidemiological clue. The biphasic illness typically begins with leptospiremic phase (fever, myalgia, conjunctival injection) followed by immune phase with renal involvement (hematuria, jaundice from hepatorenal syndrome). Microscopic agglutination test (MAT) is the gold standard and reference serological test for leptospirosis diagnosis. It detects IgM and IgG antibodies against live leptospiral serovars with high specificity (>95%) and becomes positive from day 5–7 of illness—precisely when this patient presents. MAT can identify the infecting serovar, guiding epidemiological investigation. While culture from blood/CSF is possible in the first week, it requires specialized media (Fletcher's or EMJH) and is technically demanding in routine labs. MAT remains the investigation of choice in clinical practice across Indian tertiary centers and is recommended by the National Centre for Disease Control (NCDC) guidelines for leptospirosis confirmation.

Why the other options are wrong

B. Paul-Bunnell test — Paul-Bunnell (heterophile antibody) test is specific for infectious mononucleosis caused by EBV, not leptospirosis. While EBV can cause jaundice and hepatitis, it does NOT cause conjunctival injection without exudate or hematuria. This is an NBE trap pairing a serological test with a systemic febrile illness to confuse students unfamiliar with leptospirosis serology. C. Widal test — Widal test detects antibodies against Salmonella typhi and Salmonella paratyphi, used for enteric fever diagnosis. Although enteric fever presents with fever and jaundice, it does NOT cause conjunctival injection or hematuria. Sewage exposure and the specific constellation of signs point to leptospirosis, not typhoid—a common NEET trap conflating occupational exposure with enteric fever. D. Weil-Felix reaction — Weil-Felix is a non-specific heterophile agglutination test for rickettsial infections (spotted fever, typhus). While rickettsiae cause fever and rash, they do NOT cause the triad of conjunctival injection + hematuria + jaundice seen here. This option exploits confusion between rickettsial and leptospiral serology—both are zoonotic, but Weil-Felix is obsolete and insensitive for leptospirosis.

High-Yield Facts

Mnemonics

LEPT for Leptospirosis Clues Labor/occupational (sewage, water), Eye (conjunctival injection), Pain (abdominal), Test (MAT). Use when you see a worker with fever + conjunctival injection + hematuria. MAT = Gold Standard for Leptospirosis Microscopic Agglutination Test; remember it detects antibodies from day 5–7 and identifies the serovar—no other test does both.

NBE Trap

NBE pairs "sewage worker + fever + jaundice" with Widal test (enteric fever) and Weil-Felix (rickettsial disease) to trap students who recognize occupational exposure but confuse the causative organism. The specific triad of conjunctival injection + hematuria + jaundice is the discriminator for leptospirosis.

Clinical Pearl

In Indian sewage and agricultural workers, leptospirosis is endemic and often misdiagnosed as enteric fever or dengue. The absence of rash and presence of conjunctival injection without exudate clinches leptospirosis; MAT confirmation within 5–7 days guides early doxycycline therapy to prevent progression to Weil's disease (fulminant hepatorenal failure).

_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology Ch. 27 (Leptospira); Harrison's Principles of Internal Medicine Ch. 164 (Leptospirosis)_