HIV Laboratory Diagnosis

Diagnostic Algorithm for HIV Testing

The Centers for Disease Control and Prevention recommends a fourth-generation antigen/antibody combination assay as the initial test for HIV laboratory diagnosis, which simultaneously detects HIV p24 antigen and HIV antibodies 1, 2
Fourth-generation HIV antigen/antibody combination assays are recommended as the initial screening tests for diagnosis of HIV infection, detecting both HIV antibodies and p24 antigen, allowing for earlier diagnosis of infection (4-7 days after detectable virus by NAAT) 1, 2
These assays have a sensitivity and specificity greater than 99.5% 3

Positive screening tests must be confirmed with additional testing, including HIV-1/HIV-2 antibody differentiation immunoassay to distinguish between HIV-1 and HIV-2 infections 1
If the differentiation assay is negative, nucleic acid amplification testing (NAAT) is recommended to rule out acute HIV-1 infection, with a mean reduction of the window period by approximately 4 days compared to third-generation antibody-only tests 1, 4

For persons with recent high-risk exposure (within 72 hours), rapid HIV antibody testing should be performed, and if testing is performed in the context of post-exposure prophylaxis (PEP) evaluation, both a laboratory-based antigen/antibody test and an HIV NAT are recommended 2, 4, 5, 6
For infants born to HIV-positive mothers, definitive diagnosis should be based on laboratory evidence of HIV in blood or tissues by culture, nucleic acid, or antigen detection rather than antibody testing 7

For persons who have taken PEP, follow-up testing should include both a laboratory-based antigen/antibody test and a diagnostic NAT 4-6 weeks after PEP initiation, and high-risk individuals should be tested at least annually, while those at very high risk should be tested every 3 months 2, 4, 5, 6

The American Academy of Family Physicians recommends that patients presenting within 72 hours of potential exposure undergo rapid HIV antibody testing immediately, and also obtain both a laboratory-based antigen/antibody test and an HIV NAAT 8
The Centers for Disease Control and Prevention suggests that HIV-2 testing should be considered when the patient is from or has sex partners from West Africa or other HIV-2 endemic regions 9

For individuals currently taking or recently exposed to antiretrovirals, the Centers for Disease Control and Prevention recommends performing both laboratory-based antigen/antibody test and diagnostic NAAT 8
For infants under 15-18 months of age, the Centers for Disease Control and Prevention advises against using standard antibody testing, as maternal antibodies cross the placenta and will cause false-positive results, and instead recommends definitive diagnosis requires laboratory evidence of HIV in blood or tissues by culture, nucleic acid testing, or antigen detection 9