Testing associated with HIV/AIDS involves detecting HIV antigen (p24) and/or the affected person's response to HIV (antibodies), measuring the amount of virus, or detecting the viral nucleic acid. The goals of HIV testing are to:
- Screen for and diagnose HIV infection
- Measure and monitor the amount of virus in the person's blood (the viral load)
- Evaluate HIV's resistance to available drug therapies
HIV testing is the only way for someone to know if he or she has HIV infection. Early detection and treatment of HIV infection and immune system monitoring can greatly improve long-term health. Also, if a person knows his or her HIV status, it may help change behaviors that can put that person and others at risk.
Several organizations recommend routine screening for HIV. The Centers for Disease Control and Prevention (CDC), American College of Physicians (ACP), and the U.S. Preventive Services Task Force (USPSTF) recommend that anyone between the ages of 13 and 64 (or 15 to 65 in the case of the USPSTF) and pregnant women be screened for HIV at least once.
Certain individuals should get tested at least once to learn their status, even if they are not between the ages of 13 and 64. These include:
- People diagnosed with hepatitis, tuberculosis (TB) or a sexually transmitted disease
- People who received a blood transfusion between 1978 and 1985 or had a sexual partner who received a transfusion and later tested positive for HIV
- A healthcare worker with direct exposure to blood on the job
- Any individual who thinks they may have been exposed
Screening at least annually is advised for those at high risk for HIV and is recommended when an individual:
- Has had unprotected sex with more than one partner since the last HIV test
- Is a man who has had sex with another man (CDC suggests that gay or bisexual men may benefit from more frequent screening, such as every 3 to 6 months)
- Has used street drugs by injection, especially when sharing needles and/or other equipment
- Has exchanged sex for drugs or money
- Has an HIV-positive sex partner
- Has had sex with anyone who falls into one of the categories listed above or is uncertain about their sexual partner's risk behaviors
Different types of tests may be used for HIV screening:
- Combination HIV antibody and antigen test—this is the recommended screening test for HIV. It is available only as a blood test. It detects the HIV antigen called p24 plus antibodies to HIV-1 and 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.) The level of p24 antigen and the amount of virus (viral load) increase significantly soon after initial infection. Testing for p24 allows for detection of early infections, before HIV antibody is produced. About 2-8 weeks after exposure, antibodies to HIV are produced in response to the infection and remain detectable in the blood thereafter, making the antibody test useful for detecting infections weeks after exposure. By detecting both antibody and antigen, the combination test increases the likelihood that an infection is detected soon after exposure.
- 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. These tests are available as blood tests or tests of oral fluid.
- p24 antigen testing—this is used alone without the antibody test only in rare cases when there is a question about interference with an HIV antibody test.
There are a few different ways a person can get access to HIV screening:
- A blood or oral sample can be collected in a health practitioner's office or a local clinic and sent to a laboratory for testing. Certain testing centers provide either anonymous (the name is never given) or confidential (the name is given but kept private) HIV testing and counseling. People can also contact their state, county, or city health department to find out where testing may be available. To find a testing site near you, visit the National HIV and STD Testing Resources.
- In these same settings, there may be a rapid test available in which results are generated in about 20 minutes.
- A home collection kit is available that allows a person to take a sample at home and then mail it to a testing center. Results are available over the phone, along with appropriate counseling.
- There is a home test for HIV test that uses an oral sample and results are available in about 20 minutes. This allows the person tested to remain anonymous and to get confidential results. The home test has two limitations: 1) testing on oral fluid is less sensitive than a blood test so the home test may miss some cases of HIV that a blood test would detect; and 2) the home test is not as accurate when it is performed at home by a lay person compared to when it is performed by a trained health professional. However, the convenience of home testing might encourage some people who might otherwise be reluctant to go to a health practitioner or clinic to learn their HIV status.
If any of the above screening tests is positive, then it must be followed by a second test to establish a diagnosis. This second test is an antibody test that is different than the first test. If the second test does not agree with the first test, then a third test is performed that detects the genetic material (RNA) of the virus.
In 2014, the CDC updated its HIV testing recommendations to include a new testing protocol, which has been accepted by the Clinical Laboratory Standards Institute (CLSI):
- Screen for HIV using a combination HIV antigen/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.
New HIV screening algorithm. Courtesy of James Faix, MD
Two tests once commonly used to test for HIV, HIV-1 Western blot and HIV-1 immunofluorescence assay, are not included in this new protocol and should not be used since these tests detect antibody later in the infection (around 28 days) and may give a false-negative result.
- HIV viral load testing—measures the amount of HIV in the blood; it is performed when a person is first diagnosed to help determine the status of the disease and is ordered at intervals to monitor the effectiveness of therapy.
- CD4 count—measures the number of CD4 T-cells in the blood; it is ordered when a person is first diagnosed to get a baseline assessment of the immune system and done at intervals to monitor therapy and the status of the immune system. If a person is doing well on treatment, this test may be done yearly.
- HIV genotypic resistance testing—ordered when someone is initially diagnosed to determine whether the particular strain(s) of HIV that the person has is resistant to certain antiretroviral drug therapies; also ordered when treatment is changed or when there is evidence of treatment failure.
- Phenotypic resistance testing–sometimes ordered for 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.
- Individuals 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. (Read more in the article on Pharmacogenetic Tests.)
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 health and organ function. Some examples include:
- Complete blood count
- Comprehensive metabolic panel
- Tests for other sexually transmitted diseases, such gonorrhea or syphilis
- Tests for other infections such as viral hepatitis or tuberculosis
Testing such as a chest X-ray or an imaging scan may sometimes be performed to help evaluate the person's health status.