## Initial HIV Screening Strategy **Key Point:** The 4th generation antigen-antibody combination ELISA is the gold standard initial screening test for HIV in India and globally, as per WHO and NACO guidelines. ### Why 4th Generation ELISA? **High-Yield:** 4th generation tests detect: - HIV-1 and HIV-2 antibodies - p24 antigen (detectable 2–4 weeks post-infection) - Reduces the diagnostic window compared to 3rd generation (antibody-only) tests - Sensitivity and specificity both >99% in established infection ### Testing Algorithm in India | Step | Investigation | Purpose | |------|---------------|----------| | 1 | 4th gen ELISA (screening) | Initial detection; high sensitivity | | 2 | Rapid test or ELISA (different kit) | Confirmation if 1st positive | | 3 | Western blot or HIV-1/2 differentiation ELISA | Confirmatory; resolves discordant results | | 4 | HIV RNA PCR | If acute infection suspected (window period) or confirmation failure | **Clinical Pearl:** In acute retroviral syndrome (fever, rash, lymphadenopathy, oral candidiasis within weeks of exposure), p24 antigen may be detectable before antibodies appear—making 4th generation ELISA ideal for early detection. ### Timing Considerations **Mnemonic: ELISA Window — Antibody Detection Timeline** - **E**arly: p24 antigen appears at 2–4 weeks - **L**ate: IgM antibodies at 3–4 weeks - **I**ncreasing: IgG antibodies at 4–6 weeks - **S**table: Both antibodies persist; antigen may decline - **A**ssay: 4th gen captures all three markers [cite:NACO National Guidelines on HIV Testing 2017; Park 26e Ch 7]
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