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Laboratory tests for HIV infection

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The laboratory diagnosis of HIV infection is usually made on the basis of serology - detection of HIV antibody i.e. the detection of HIV-1/2 antibodies or simultaneous detection of HIV-1/2 antibodies and HIV-1 p24 antigen

most widely used means of diagnosing HIV
results are reported as non-reactive or reactive
generally classified as either
- first-line assays (sometimes referred to as screening assays)
  - can provide the presumptive identification of reactive specimens and thus should have superior sensitivity e.g -
    - enzyme linked immunosorbent assay (ELISA)/enzyme immuno assay (EIA) test
    - rapid HIV tests/point-of-care tests (POCTs)
      - oral fluid or pin prick blood samples can be used
      - results can be given within minutes of the specimen being taken
      - specificity is lower than laboratory tests
      - all “reactive” POCT tests should be confirmed with a conventional blood test
    - simple assays e.g. – combo immunoassays and particle or latex agglutination assays that detect the presence of HIV-1/2 antibodies and/or HIV-1 p24 antigen.
- second or third-line assays (sometimes referred to as supplemental assays, or confirmatory assays)
  - a combination of rapid HIV tests, simple assays and EIAs can be used as second- and third-line assays to confirm an initial reactive test result
  - in addition, line immunoassays (LIAs), based on recombinant proteins and/or synthetic peptides capable of detecting antibodies to specific HIV-1 and/or HIV-2 proteins, have been widely used to confirm HIV infection.
    - line immunoassays have replaced Western blotting n many settings and serve a similar purpose (1,2)

In addition HIV infection can also be diagnosed by detecting the presence of the virus itself.

presence of the virus itself
- p24 antigen
  - 4th generation combination test is used to detect p24 antigen
presence of its genetic material (DNA PCR, viral load)
- RNA/DNA polymerase chain reaction tests
  - these quantitative assays are a form of Nucleic Acid Amplification Test (NAAT)
  - not often used as an initial diagnostic test for HIV in adults as they are expensive and can risk a high false positive rate in the absence of laboratory interpretation
  - preferred test of specialists if primary HIV infection is suspected and the antibody/antigen test is negative
- viral load
  - primarily used for monitoring antiretroviral treatment, but have been validated by many labs as a supplemental test during the window period of HIV infection (2,3)

Patients who identify a specific risk occurring more than 4 weeks previously should be offered a 4th generation HIV test immediately without waiting for 3 months (12 weeks).

a negative test will exclude HIV infection in majority of patients
additional test should be offered at 3 months (12 weeks) to all patients to exclude HIV infection definitively
low risk patients may wait for 3 months to avoid doing the test twice (4)

HIV antibody tests are used for diagnosis and screening.

Reference:

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Laboratory tests for HIV infection

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