A 20-year-old student from Delhi develops fever, severe headache, retro-orbital pain, myalgia and a maculopapular rash on day 4. NS1 antigen is positive on day 3. Platelet count falls to 60,000/µL. Which test would confirm dengue on day 8 of illness when NS1 is no longer detectable?

• A RT-PCR for dengue RNA which remains positive until day 21
• B Widal test showing rising titres
• C Dengue IgG ELISA which is positive from day 1 in primary infection
• D IgM-ELISA (MAC-ELISA) detecting dengue-specific IgM antibody ✓

Explanation

In dengue, NS1 antigenemia is detectable from day 1–9 of illness and RT-PCR/virus isolation is possible only in the first 5 days. From around day 5–6, dengue-specific IgM antibodies appear (peak days 7–10) and remain detectable for up to 2–3 months. IgM ELISA (MAC-ELISA) is the test of choice for secondary serodiagnosis from day 5 onwards. Dengue IgG in primary infection appears after IgM (day 10+); in secondary infection IgG rises rapidly. Widal test is for enteric fever, not dengue. RT-PCR has a short window of only 5 days.

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

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Written and medically reviewed by the StethoPrep medical team.