Which HIV serological marker indicates the narrowest 'window period' for detecting acute HIV infection, even before antibodies develop?

• A Anti-HIV gp41 antibodies
• B Anti-HIV gp120 antibodies
• C Anti-HIV p17 antibodies
• D HIV p24 antigen (4th-generation combined Ag/Ab assay) ✓

Explanation

4th-generation combined HIV antigen/antibody immunoassays detect both HIV-1/2 antibodies and HIV p24 antigen, reducing the window period to approximately 18 days from infection (compared to ~22 days for 3rd-generation antibody-only assays). p24 antigen appears during acute viraemia before antibody seroconversion. Among antibodies, anti-gp41 appears earliest but still after p24. HIV RNA NAAT is the most sensitive (window ~10 days) but is not routinely used for screening. 4th-gen assays are now the WHO-recommended first-line test.

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

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Written and medically reviewed by the StethoPrep medical team.