A clinician wants to distinguish dengue fever from dengue haemorrhagic fever serologically during the acute phase (day 3 of illness). Serum NS1 antigen is positive. Which additional test best differentiates primary from secondary dengue infection at this stage?

• A Haemagglutination inhibition (HAI) titre ≥1:2560 at day 3 is diagnostic of secondary dengue
• B Plaque reduction neutralisation test (PRNT) for all four dengue serotypes
• C Complete blood count showing thrombocytopenia <100,000/mm³ is sufficient to classify secondary dengue
• D IgM/IgG ratio by ELISA — a ratio >1.2 suggests primary infection; ratio <1.2 or IgG predominance suggests secondary infection ✓

Explanation

In primary dengue (first infection), IgM rises rapidly and is the dominant antibody in the first 2 weeks — giving an IgM/IgG ratio >1.2. In secondary dengue (reinfection with a different serotype), an anamnestic response rapidly boosts IgG antibodies, so IgG rises earlier and more steeply, with the ratio <1.2 or IgG clearly predominant even by day 3–5. Secondary dengue is associated with a higher risk of dengue haemorrhagic fever/dengue shock syndrome due to antibody-dependent enhancement (ADE). HAI titres are used for convalescent-phase serodiagnosis. PRNT is the gold-standard for serotype confirmation but is labour-intensive. Thrombocytopenia alone does not distinguish primary from secondary infection.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.