From the Guidelines
For HIV confirmation, the most definitive test is the HIV RNA viral load test, which is better than the p24 antigen test. The RNA test directly measures viral genetic material in the blood and can detect HIV infection earlier (as soon as 10-14 days after exposure) and with greater sensitivity than p24 testing 1. While p24 antigen testing can detect early infection, it becomes less reliable after antibodies develop. The gold standard for HIV diagnosis typically follows a sequence: initial screening with a 4th generation antibody/antigen test, followed by confirmation using an HIV RNA viral load test if needed. Other confirmatory tests include Western blot and immunofluorescence assays, but these have largely been replaced by more sensitive molecular methods 1.
Some key points to consider when choosing a test for HIV confirmation include:
- The sensitivity and specificity of the test: HIV RNA viral load tests have high sensitivity and specificity, making them a reliable choice for diagnosis 1.
- The stage of infection: In acute HIV infection, HIV RNA viral load tests can detect infection earlier than p24 antigen tests or antibody tests 1.
- The need for confirmatory testing: A positive result from a screening test should be confirmed with a more specific test, such as an HIV RNA viral load test 1.
For definitive diagnosis, healthcare providers often use a combination approach, starting with screening tests and then confirming positive results with viral load testing, which not only confirms infection but also provides important information about disease progression and treatment effectiveness.
From the Research
HIV Confirmation Tests
The choice of test for HIV confirmation depends on various factors, including the stage of infection and the individual's immune response.
- p24 antigen test: This test detects the presence of the p24 protein, which is a component of the HIV virus. It is useful for detecting early HIV infection, before the body has produced antibodies against the virus 2, 3, 4, 5.
- RNA load test: This test measures the amount of HIV RNA in the blood, which can indicate the level of viral activity. It is useful for monitoring the effectiveness of antiretroviral therapy and detecting early HIV infection 6.
- Other tests: Fourth-generation antigen/antibody combination tests, which detect both p24 antigen and HIV antibodies, are also available and have been shown to be effective in detecting early HIV infection 2, 3, 4.
Comparison of Tests
Studies have compared the performance of different HIV tests, including p24 antigen tests, RNA load tests, and fourth-generation antigen/antibody combination tests.
- A study published in 2021 found that the p24 antigen test was able to detect 4 cases of acute HIV infection with high viral loads, while the HIV antibody test was still negative 2.
- A study published in 2018 found that fourth-generation antigen/antibody combination tests performed similarly to p24 antigen tests in detecting diverse HIV subtypes, but ultrasensitive p24 antigen tests achieved dramatically increased sensitivities 3.
- A study published in 2020 found that the new version of the Determine™ HIV-1/2 Ag/Ab Combo rapid diagnostic test had increased sensitivity to detect HIV-1 p24CA-positive, immunoblot-negative plasma samples compared to the old version 4.
Limitations and Future Directions
While p24 antigen tests and RNA load tests are useful for detecting early HIV infection, there are limitations to their use, including the need for specialized equipment and trained personnel.
- A review published in 2018 highlighted the challenges of detecting ultra-low levels of p24 at the point of care and identified knowledge gaps and barriers to the development of p24 diagnostics 5.
- A study published in 2010 discussed the public health rationale for rapid nucleic acid or p24 antigen tests for HIV and suggested priorities in operations research to pave the way for the future introduction of direct rapid viral testing technologies in the HIV testing marketplace 6.