In HIV-1 diagnostic testing, a reactive 4th-generation HIV Ag/Ab combination test should be followed by which confirmatory algorithm per current guidelines?

• A Western blot for HIV-1 and HIV-2 antibodies
• B p24 antigen-only ELISA to confirm p24 positivity
• C HIV-1/HIV-2 antibody differentiation immunoassay, then HIV-1 NAT if discordant ✓
• D CD4 count measurement to determine immunological stage

Explanation

Current CDC and WHO-recommended HIV testing algorithm (2014 update) follows a reactive 4th-generation Ag/Ab combination test with an HIV-1/HIV-2 antibody differentiation immunoassay (Step 2). If Step 2 is negative or indeterminate (suggesting acute HIV with only p24 antigen detected), an HIV-1 NAT (NAAT) is performed as Step 3 to detect early/acute infection. Western blot is no longer recommended as the primary confirmatory test because it misses early infections with p24-only reactivity and has lower specificity than the new algorithm.

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

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