Question
Discuss the laboratory diagnosis of a patient suspected of having HIV infection.
Answer
Laboratory diagnosis of suspected HIV infection
1. Screening tests:
- ELISA (Enzyme-Linked Immunosorbent Assay) — detects HIV antibody (and often combined antigen-antibody in 4th-generation assays, detecting p24 antigen plus antibody to shorten the window period); high sensitivity, used as the initial screening test.
- Rapid tests — point-of-care immunochromatographic strip tests, providing results within 15–30 minutes; widely used in field/outreach settings and per NACO’s national testing algorithm.
2. NACO testing strategy (India): as per national guidelines, a sample reactive on an initial test (Strategy I/II/III context-dependent) must be confirmed using two more different rapid/ELISA test kits (using different antigen preparations/test principles); a sample is called HIV-positive only if it is reactive on all three different tests (Strategy III, used for individual diagnosis).
3. Confirmatory/supplemental tests:
- Western blot — detects antibody to individual HIV structural proteins (gp160, gp120, gp41, p24, etc.), traditionally used as the gold-standard confirmatory test in many settings (though largely superseded by the NACO three-test strategy in India).
- Line immunoassay (LIA) — similar principle to Western blot, using recombinant/synthetic antigens on a strip.
4. Molecular/virological tests:
- HIV RNA PCR (viral load) — quantifies plasma HIV RNA, useful for early diagnosis (during the window period, before antibody seroconversion), monitoring treatment response, and diagnosing infants born to HIV-positive mothers (since maternal antibody crosses the placenta, making antibody tests unreliable in infants <18 months).
- Proviral DNA PCR — detects integrated HIV DNA in host cell genome, also used in early infant diagnosis.
5. CD4 count: not diagnostic of HIV infection itself, but essential for staging the disease and guiding treatment/monitoring (WHO/NACO clinical staging, initiation of ART, opportunistic infection risk assessment).
6. p24 antigen assay: detects viral core protein, appearing before antibody, useful in early infection/window-period detection (incorporated into 4th-generation combo ELISA).
Summary of typical algorithm: any reactive screening test → repeat with a second, different test kit → if reactive, confirm with a third, different test kit (as per NACO Strategy III) → HIV-positive status confirmed only when reactive on all three; CD4 count and viral load performed subsequently for staging and monitoring, not for diagnosis itself.