Testing associated with HIV/AIDS involves either measuring the affected person's response to HIV (antibodies) or measuring and evaluating the virus itself. Most tests are performed using the person's blood. The goals of HIV testing are to:
- Screen for and diagnose HIV infection – confidential or anonymous testing is available for those who think they may have been exposed. Initial tests often involve testing a small sample of blood from a fingerstick or an oral sample for HIV antibodies produced by the person's immune system. Results are often available within minutes; however, a positive screening result needs to be confirmed by another test.
- Measure and monitor the amount of virus in the person's blood (the viral load)
- Evaluate and monitor the person's immune function status
- Evaluate HIV's resistance to available drug therapies
Different types of tests may be used for HIV screening:
- HIV antibody testing—all HIV antibody tests used in the U.S. detect HIV-1, and some tests have been developed that can also detect HIV-2. (HIV-1 is the most common type found in the United States, while HIV-2 has a higher prevalence in parts of Africa.)
- p24 antigen testing—this test is one of the earlier tests developed to detect HIV infection. Because levels of p24 antigen fall as antibody develops and do not rise again until late in the course of HIV infection, it may only be detectable during a short window of time after exposure, so its utility is limited as an individual test. Development of newer, better technology such as nucleic acid amplification test (NAAT) has resulted in a decline in p24 antigen testing. However, this test is now being used more effectively as part of a combination test with HIV antibody. By detecting both antibody and antigen, the combination test increases the likelihood that an infection is detected soon after exposure.
A screening test must be followed by a second test to establish a diagnosis. With the aim of improving the diagnostic process, a new testing protocol has been proposed and is in the process of being implemented. Over time, more clinics and doctors may opt to follow these steps:
- Screen for HIV using an HIV antigen/antibody test or an HIV antibody test; then
- Verify a positive with a second HIV antibody test that differentiates between HIV-1 and HIV-2;
- If results between the first and second test do not agree, then the next test to perform is an HIV-1 RNA test (nucleic acid amplification test, NAAT). If the HIV-1 RNA is positive, then the test is considered positive.
- HIV viral load testing—measures the quantity of HIV in the blood. Ordered to help decide when to start therapy and ordered at intervals to monitor the effectiveness of therapy.
- CD4 count—measures the number of CD4 T-cells in the blood. Ordered at intervals to determine when to start HIV therapy. Also ordered to monitor therapy, HIV progression, and the status of the immune system.
- HIV genotypic resistance testing—ordered to determine whether the particular strain(s) of HIV that the person has is resistant to available antiretroviral drug therapies. Ordered when someone is initially diagnosed to determine if the virus is resistant to the commonly prescribed antivirals. Also ordered when treatment is initiated, changed, and when there is evidence of treatment failure.
- Phenotypic resistance testing–sometimes ordered on those who are resistant to multiple antiretroviral drugs to help guide treatment. This test evaluates whether the person's strain(s) of HIV can be inhibited by various concentrations of antiretroviral drugs.
A number of other laboratory testing may be performed as part of overall care of an HIV-infected individual. Several tests may be done to identify and monitor the treatment of opportunistic infections, complications, and drug toxicities. Testing may also be ordered at intervals to evaluate the person's state of health and organ function. Some tests are ordered to evaluate risks associated with using a specific HIV treatment. Those who are planning to take the drug abacavir may be tested first for the gene allele, HLA-B*5701. If they are positive for it, they are at an increased risk of having a potentially severe hypersensitivity reaction and another drug should be considered.
Testing such as a chest x-ray or an imaging scan may sometimes be performed to help evaluate the person's health status.