During a dengue outbreak, a patient on day 6 of illness has defervescence but develops severe abdominal pain, pleural effusion on chest X-ray and platelet count 18,000/µL. NS1 antigen test is now negative. Which serological test would be most useful at this stage?

• A RT-PCR for dengue viral RNA
• B NS1 antigen test repeated
• C Plaque reduction neutralisation test (PRNT)
• D IgM antibody detection by MAC-ELISA ✓

Explanation

NS1 antigen is detectable predominantly in the first 1–5 days of febrile illness and declines as antibody responses mount. By day 6 after defervescence, NS1 sensitivity drops sharply. IgM antibodies (detected by MAC-ELISA) appear by days 3–5 and peak around day 10, making them the optimal marker at this stage. RT-PCR is most sensitive in the first 5 days of viraemia. PRNT is the serotype-specific gold standard for seroepidemiological surveys but is not used for acute patient diagnosis.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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