A 28-year-old woman returns from Southeast Asia with 5 days of fever, severe headache, retro-orbital pain, and myalgia. She develops spontaneous gum bleeding on day 5. CBC shows: WBC 2800/µL, Hb 12 g/dL, platelets 38,000/µL. NS1 antigen is positive. Dengue NS1 is detectable at what stage of illness?
- A Days 1–5 of illness (early viraemic phase, before antibody seroconversion) ✓
- B Only after day 7 when IgM antibodies appear
- C Only during the convalescent phase (days 10–14)
- D NS1 persists positive for 3 months after infection
Correct answer: A. Days 1–5 of illness (early viraemic phase, before antibody seroconversion)
Explanation
Dengue NS1 (non-structural protein 1) antigen is secreted into the bloodstream during active dengue viraemia and is detectable from day 1 through approximately day 5–7 of illness, before significant antibody titres develop. It is the preferred early diagnostic marker for acute dengue. IgM antibodies appear from day 3–5 and peak at day 14–21 (primary infection) or earlier in secondary infection. NS1 is not detectable in the late or convalescent phase.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.