HIV Testing in the Laboratory
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Direct answer
Three reactive tests, each using a different antigen preparation or format, are required before an HIV-positive diagnosis is reported in India: the NACO strategy (per the National Guidelines for HIV Testing) applies a highly sensitive first assay, then a second and third assay of different antigens or principles, with concordant reactivity in all three constituting a positive report — and any discordance triggering repeat sampling and referral rather than a verdict. Assay generation governs the window period: third-generation antibody ELISAs detect IgG and IgM from about three weeks, fourth-generation antigen-antibody combos shorten this to roughly two to three weeks by adding p24 antigen detection, and nucleic acid testing detects infection earliest of all. Blood banks separately screen every donation with one highly sensitive test and discard reactive units without pursuing confirmation, while integrated counselling precedes and follows every diagnostic test.
What you must remember
- NACO strategy III structure: test 1 (sensitive screen — ELISA or rapid), test 2 (different antigen/format), test 3 (again different) — all three reactive equals positive; discordant samples are retested and referred, never averaged into a diagnosis.
- Blood-bank rule (Strategy I): a single highly sensitive assay screens donations; reactive units are discarded and donors referred — confirmation is not the blood bank's job.
- Window periods: third-generation antibody assays roughly 21-22 days; fourth-generation combo about 18 days by adding p24 antigen; NAT around 7-11 days — the examinable descending ladder.
- Assay designs: third generation — indirect ELISA for anti-HIV IgG and IgM; fourth generation — sandwich ELISA detecting p24 antigen plus antibody simultaneously; rapid tests — immunochromatographic, whole-blood friendly, field use.
- Infant diagnosis: maternal IgG crosses the placenta and confounds all antibody tests up to 18 months, so HIV-exposed infants are diagnosed by DNA PCR (early infant diagnosis, with a dried blood spot protocol at the programme's specified ages).
- CD4 role: universal "test and treat" means CD4 no longer gates therapy start, but baseline CD4, with 6-monthly monitoring and a count below 200 defining significant immunosuppression and prophylaxis need, remains laboratory work (flow cytometry).
- Ethics layer: universal precautions, post-exposure prophylaxis protocol, coded samples, and results disclosed only after counselling.
Running the NACO algorithm
Walk a walk-in client through an integrated counselling and testing centre. Pre-test counselling done, consent recorded, a rapid test runs first: reactive. The counsellor holds all interpretation — no result is spoken yet. Test 2, a different format and antigen mix, is run: reactive again. Test 3: reactive. Three of three concordant — reported HIV positive, post-test counselled, linked to antiretroviral therapy under test-and-treat, with baseline CD4 and viral load ordered. Had test 2 been non-reactive, the results are not averaged: the sample is retested and discordance goes to the reference laboratory — at low prevalence a single false reactive is likelier than infection.
Now the blood-bank contrast. A donation screens reactive on a single ELISA. The unit is discarded and the donor referred — but the donation is not "confirmed HIV positive"; that belongs to the counselling pathway. Keeping the diagnostic algorithm and the screening rule straight is exactly the discrimination examiners probe. The final programme layer is infant testing: a nine-month-old exposed infant gets a dried blood spot DNA PCR; antibody tests on that child read positive whether infected or merely exposed — the most examinable exception to adult serology.
Counselling and documentation angles
Viva questions here are about conduct: informed consent before testing, post-test counselling and ART linkage with a positive report, and never results without counselling or consent. The technical companions: which assay shortens the window most (fourth-generation combo, then NAT), why Western blot lost primacy (the three-test algorithm replaced it), and the needle-stick response — washing, source testing, post-exposure prophylaxis within hours.
Frequently asked questions
What is the NACO three-test strategy for HIV diagnosis?
Three sequential assays using different antigens and formats; reactivity in all three concordantly is reported as positive, and discordant results are repeated and referred for resolution.
How does a fourth-generation HIV assay differ from a third-generation one?
Third-generation assays detect antibody only, whereas fourth-generation combos detect p24 antigen and antibody together, shortening the window period to roughly two to three weeks.
Why are antibody tests unreliable in infants below 18 months?
Passively transferred maternal IgG makes all antibody tests positive regardless of infection status, so early infant diagnosis uses HIV DNA PCR on blood or dried blood spots.
How is donor blood screened for HIV in Indian blood banks?
With a single highly sensitive assay; a reactive unit is discarded and the donor referred for counselling and confirmatory testing through the NACO pathway.
What is the role of CD4 counting under test-and-treat?
CD4 no longer decides when to start ART, but baseline and periodic CD4 counts (below 200 marking significant immunosuppression) guide opportunistic-infection prophylaxis and immune monitoring.