Recommended Test for HIV Antigen Detection
For high-risk adults and adolescents, use a fourth-generation combination HIV antibody/antigen (p24) immunoassay as the initial screening test, which detects both HIV antibodies and p24 antigen simultaneously. 1
Primary Testing Recommendation
The combination HIV antibody and p24 antigen test is the preferred screening method because it can detect HIV infection earlier than antibody-only tests, reducing the diagnostic window period by detecting p24 antigen during acute infection before antibodies develop. 1, 2
Fourth-generation combination assays have sensitivity and specificity both greater than 99.5% and can detect acute HIV infection 10 days earlier than third-generation antibody-only tests. 1, 3
The International Antiviral Society-USA Panel specifically recommends screening with assays that can detect recent HIV infection, explicitly naming the combined HIV antibody and antigen test as optimal. 1
When to Use HIV RNA Testing
HIV RNA testing (plasma RNA assay using PCR or bDNA) should be added when acute HIV infection is clinically suspected, particularly in patients with compatible symptoms (flu-like illness) and recent high-risk behavior. 1, 2, 4
The CDC emphasizes that when acute retroviral syndrome is a possibility, a plasma RNA test must be used in conjunction with an HIV antibody test to diagnose acute HIV infection, as antibody tests may still be negative during this critical period. 1, 2
HIV RNA can be detected by sensitive PCR or bDNA assays together with a negative or indeterminate HIV antibody test, providing laboratory evidence of acute HIV infection. 2
Testing Algorithm for High-Risk Populations
Initial screening:
- Use fourth-generation combination HIV antibody/p24 antigen immunoassay for all routine screening. 1
If acute infection suspected (symptoms + recent exposure):
- Order HIV RNA quantitative test (also called "HIV viral load" or "plasma HIV RNA level") in addition to the combination antibody/antigen test. 2, 5
- Do not wait for test results if clinical suspicion is high—refer immediately to HIV specialist. 2
Confirmatory testing:
- Initial positive results on combination assay require confirmation with Western blot or immunofluorescent assay. 1
Critical Clinical Pitfall
Never rely on antibody-only tests (third-generation) in high-risk patients or those with potential acute infection, as these miss the critical window period when viral loads are highest and transmission risk is greatest. 2, 6
The routine use of direct virus assays (HIV RNA) is specifically recommended when acute HIV infection is suspected, despite not being recommended for routine screening in all patients. 2
Fourth-generation combination assays detect 80-89% of acute infections that would be missed by antibody-only testing. 7, 8
Screening Frequency for High-Risk Groups
Men who have sex with men, transgender persons, people who inject drugs, and those newly diagnosed with other STIs should be tested every 3 months as long as risk continues. 1
The CDC recommends at least annual screening for persons at very high risk, including injection drug users, those with multiple sex partners, and those in high-prevalence settings. 1