HIV Laboratory Diagnosis and NACO Algorithm
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Direct answer
HIV laboratory diagnosis detects antibodies, antigens or viral nucleic acid, and India's National AIDS Control Organisation (NACO) uses a serial algorithm in which, for asymptomatic ICTC clients, three different assays must all be reactive before a positive result is reported. Discordant results are reported as indeterminate and repeated after 2 to 4 weeks. Infants born to HIV-positive mothers cannot be diagnosed by antibody tests and need HIV DNA PCR instead.
What you must remember
- Window period: The interval between infection and detectable antibodies, shortened by fourth-generation assays that also detect p24 antigen.
- ELISA generations: Early tests used whole-virus lysate, later ones used recombinant antigens and sandwich formats, and the fourth generation combines p24 antigen and antibody detection.
- Rapid tests: Point-of-care antibody tests used for screening in counselling and testing centres, and a reactive result always needs confirmation.
- Western blot: A confirmatory test showing bands against p24, gp41 and gp120 or gp160, with indeterminate patterns requiring follow-up.
- Nucleic acid tests: HIV RNA PCR detects acute infection during the window period, and HIV DNA PCR is the test used for infants.
- Maternal antibody: Maternal IgG crosses the placenta and can persist in infants for up to about 18 months, making antibody tests uninformative in early life.
- ICTC: Integrated Counselling and Testing Centres deliver NACO testing with pre-test and post-test counselling.
- Strategies: Strategy III (three tests A1, A2, A3) is used for asymptomatic clients at ICTCs and PPTCT centres, while Strategy 2B (two tests plus a tie-breaker) is used only for symptomatic suspected AIDS cases.
Working through the NACO algorithm
The NACO algorithm is a serial strategy, so each step either confirms a positive result or resolves a discordant one. The steps below follow the adult pathway.
- Screen with a rapid test: A non-reactive rapid test 1 result is reported as negative, but it should be repeated if exposure was recent enough to fall within the window period.
- Add a second and third assay: A reactive first test is followed by A2 and A3, each based on a different principle or antigen, with A1 the most sensitive and A2 and A3 the most specific.
- Reconcile discordant results: Two reactive tests with a non-reactive third are reported as indeterminate, and testing is repeated on a fresh sample after 14 to 28 days.
- Report and counsel: Only A1, A2 and A3 all reactive is reported as HIV-positive with post-test counselling, and the patient is referred for CD4 count assessment and antiretroviral care.
- Use molecular tests when indicated: Acute infection, infants and unresolved discordant cases are referred for nucleic acid testing.
Where the exam tries to catch you
HIV laboratory questions usually test the window period, the maternal antibody trap and the confirmatory sequence.
- Trap: Treating a single reactive rapid test as a confirmed diagnosis. Correct: Further tests with different antigens or formats are required, and in asymptomatic clients all three assays must be reactive before a positive result is reported.
- Trap: Diagnosing HIV in a nine-month-old infant from an antibody test. Correct: Maternal antibody can persist for up to about 18 months, so HIV DNA PCR is the correct test for infants.
- Trap: Accepting a negative antibody result within weeks of exposure. Correct: The window period means a negative antibody result close to exposure needs repeat testing, and nucleic acid testing if clinically needed.
Frequently asked questions
What is the window period in HIV diagnosis?
The window period is the interval between infection and the point when antibodies become detectable by standard tests. Fourth-generation assays detecting p24 antigen shorten this interval, but a negative result close to exposure still needs repeat testing or a nucleic acid test.
Why is an antibody test not used for infants born to HIV-positive mothers?
Maternal IgG antibodies cross the placenta and can persist in infants for up to about 18 months, so an antibody test cannot distinguish maternal from infant antibody. Early infant diagnosis is therefore made with HIV DNA PCR, which detects the infant's own viral DNA.
What is the role of Western blot in HIV diagnosis?
Western blot is a confirmatory test that detects antibodies against HIV proteins such as p24, gp41 and gp120 or gp160. A pattern with the required bands confirms HIV infection, while an indeterminate pattern needs follow-up testing and possibly a nucleic acid test.
What does a discordant result mean in the NACO algorithm?
A discordant result is one in which the assays in the sequence disagree, for example A1 and A2 reactive but A3 non-reactive. It is reported as indeterminate, and the client is retested on a fresh sample after 14 to 28 days rather than being labelled positive or negative.
What does ICTC stand for in HIV testing in India?
ICTC stands for Integrated Counselling and Testing Centre, where HIV testing is offered with pre-test and post-test counselling under NACO guidance. These centres are the usual setting in which the NACO algorithm is applied to adults.