A patient presents with dengue fever. On Day 3 of illness, the dengue NS1 antigen rapid test is positive. On Day 8, dengue IgM and IgG serology is sent. Which combination correctly represents secondary dengue infection serology?

• A IgM positive only, IgG negative, detectable from Day 5
• B IgM positive from Day 3 with IgG appearing only after 3 weeks
• C High IgG titre appearing early (Day 4–5) with lower IgM, IgM/IgG ratio < 1 ✓
• D NS1 positive beyond Day 7 with IgM and IgG both negative

Explanation

In secondary dengue infection (reinfection with a different serotype), the anamnestic immune response is rapid: IgG rises sharply and early (detectable by Day 4–5, earlier and higher than in primary infection). IgM is lower and the IgM:IgG ratio is characteristically less than 1 (IgG dominates). In primary dengue, IgM appears from Day 5 and exceeds IgG for several weeks. NS1 antigen falls by Day 5–7 typically. A high early IgG is the hallmark of secondary infection.

Reference: Park's Textbook of Preventive and Social Medicine, 27th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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