Dengue fever serology: A patient presents on day 4 of fever with dengue suspicion. NS1 antigen ELISA is positive. Regarding dengue diagnostics, NS1 antigen is detectable in serum during which period?
- A Days 5–10 only, coinciding with peak IgM levels in secondary dengue
- B Days 1–5 of illness (febrile phase) when viremia is present, before IgM seroconversion ✓
- C From day 10 onwards during the recovery phase as viral clearance occurs
- D Throughout infection, irrespective of viremic phase
Correct answer: B. Days 1–5 of illness (febrile phase) when viremia is present, before IgM seroconversion
Explanation
Dengue NS1 (non-structural protein 1) antigen is detectable in serum from day 1 of illness and peaks during days 1–5 (febrile/viremic phase); it typically becomes undetectable by day 5–9 as IgM antibodies appear. NS1 ELISA is therefore the preferred early diagnostic test when PCR is unavailable. IgM antibodies appear from day 5 onwards (primary dengue) or earlier in secondary dengue. Combining NS1 antigen (early phase) with IgM/IgG serology (late phase) maximizes sensitivity. NS1 detection is not useful in the late or recovery phase.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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