A patient presents with acute febrile illness 5 days post-return from a dengue-endemic region. NS1 ELISA is negative. IgM ELISA is also negative. However, RT-PCR for dengue RNA is strongly positive. How do you reconcile these results?
- A RT-PCR is positive in the early viraemic phase (days 1–5) before antibodies develop; NS1 sensitivity declines after day 5 in secondary infection due to NS1-IgG complex formation ✓
- B The negative NS1 and IgM indicate a different arboviral infection; PCR is a false positive
- C Day 5 is too late for RT-PCR to be positive; likely lab contamination
- D Secondary infection always shows positive IgM by day 5; these are mutually exclusive results
Explanation
In dengue diagnosis, RT-PCR for dengue RNA is the most sensitive test during the viraemic phase (days 1–5). NS1 antigen sensitivity is high in primary infection (days 1–9) but falls significantly in secondary dengue infection because pre-existing anti-dengue IgG from prior infection forms NS1-IgG immune complexes, reducing free NS1 detectable by ELISA. IgM may not yet be detectable by day 5 of illness, particularly in secondary infection where IgG rises before IgM ('secondary dengue antibody response' — higher initial IgG). RT-PCR positive with negative NS1 and IgM is therefore consistent with acute early/secondary dengue and the PCR result is definitive.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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