A patient presents with dengue fever. On day 4 of illness, NS1 antigen ELISA is negative, but IgM anti-dengue antibody is positive with IgG negative. This serological profile is most consistent with:

• A Primary dengue infection in the window period
• B Secondary dengue infection with cross-reactive antibodies
• C False positive IgM from cross-reacting Zika or chikungunya infection
• D Primary dengue infection after viraemia has cleared, with nascent IgM response ✓

Explanation

In primary dengue, NS1 antigen is detectable from day 1-9 (viraemic phase) but declines as viraemia clears. IgM antibodies appear from day 4-5 onward and precede IgG (which typically rises from day 7-10 in primary infection). A negative NS1 with positive IgM and negative IgG is characteristic of primary dengue after viraemia has ended. In secondary dengue, IgG rises rapidly and steeply (often before or concurrent with IgM rise), giving an IgM:IgG ratio <1.2 on day 4-5, distinguishing secondary from primary 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.