Dengue NS1 antigen ELISA is positive on day 2 of fever. On day 7, the patient is referred for thrombocytopenia (platelets 25,000/μL) and hematocrit rise. Which serological finding confirms dengue secondary infection at this stage?
- A Positive IgM ELISA alone with absent IgG
- B Negative NS1 with positive IgM only
- C High-titer IgG detectable from day 1–2 with or without IgM (secondary response pattern) ✓
- D PCR positivity replacing serology in secondary infection
Correct answer: C. High-titer IgG detectable from day 1–2 with or without IgM (secondary response pattern)
Explanation
In secondary dengue infection, anamnestic (memory) IgG response rises rapidly to very high titres within the first 1–2 days of fever; IgM may be absent or low. In primary infection, IgM appears first and IgG rises later (after day 4–5). NS1 antigen is highest in the early febrile phase (days 1–5) and fades before secondary complications develop. PCR is most sensitive in the first 5 days but becomes unreliable by day 7 in secondary infection due to lower viremia.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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