A patient presents on day 5 of illness with fever, myalgia, and thrombocytopenia. NS1 antigen ELISA is positive but IgM is negative and IgG is negative. What is the most likely interpretation?
- A Secondary dengue infection in the IgG boost phase
- B Primary dengue infection in the viraemic phase ✓
- C False positive NS1 due to cross-reaction with other flaviviruses
- D Convalescent phase with waning NS1
Correct answer: B. Primary dengue infection in the viraemic phase
Explanation
NS1 antigen is detectable from day 1–9 of primary dengue infection before antibody production begins. In primary infection, IgM appears around day 5–7 and IgG is absent or appears later. The combination of positive NS1 with negative IgM/IgG on day 5 is classic for early primary dengue. In secondary dengue, IgG rises rapidly (anamnestic response) even before IgM. NS1 cross-reactivity with West Nile or Zika is recognised but less common in clinical testing contexts.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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