Laboratory tests for HIV infection

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:

• (1) World Health Organization (WHO) 2015. HIV assays: laboratory performance and other operational characteristics: rapid diagnostic tests (combined detection of HIV-1/2 antibodies and discriminatory detection of HIV-1 and HIV-2 antibodies): report 18.
• (2) Public Health Agency of Canada. Human Immunodeficiency Virus. HIV Screening and Testing Guide.
• (3) The Medical Foundation for AIDS & Sexual Health (MedFASH) 2016. HIV in primary care. A practical guide for primary healthcare professionals in Europe.
• (4) British HIV Association, British Association for Sexual Health and HIV · British Infection Society 2008. UK National Guidelines for HIV Testing 2008