HIV antibody and HIV antigen (p24) testing is used to screen for and diagnose HIV infections. Early detection and treatment of HIV infection and immune system monitoring can greatly improve long-term health and survival. Also, if a person knows his or her HIV status, it may help change behaviors that can put him or her and others at risk.

Screening

Different types of tests may be used for HIV screening:

• Combination HIV antibody and HIV 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. A few 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. These tests can detect HIV infections in most people by 2-6 weeks 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. HIV antibody tests can detect infections in most people 3-12 weeks after exposure.
• 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 webpage.
• In these same settings, there may be a rapid test available, with which results are generated in 20 minutes or less.
• 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 that uses an oral sample and results are available in about 20 minutes. 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 healthcare professional. However, the convenience of home testing might encourage some people who might otherwise be reluctant to go to a healthcare practitioner or clinic to learn their HIV status.

Diagnosis

If any one 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. An HIV RNA test will detect HIV in most people by 1-4 weeks of infection.

Several organizations recommend routine screening for HIV:

• The Centers for Disease Control (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.
• The CDC and American College of Obstetricians and Gynecologists (ACOG) recommend that all pregnant women be screened. Repeat testing in the third trimester may be done for women at high risk. A woman who wants to make sure she is not infected with HIV before getting pregnant may opt to get tested (see Pregnancy: HIV.)
• The American Academy of Pediatrics (AAP) advises that all sexually active youth be screened, and that youths between 16 and 18 years old who live in high risk areas (areas where more than 1 in 1,000 people have HIV) be offered HIV testing at least once, regardless of sexual history.

For additional details on screening recommendations, see the articles for Teens, Young Adults, Adults, and Adults 50 and Up.

Annual screening 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

Certain individuals should get at least a one-time test, even if they are not between the ages of 13 and 64, and learn their status. These include:

• People diagnosed with hepatitis B or hepatitis C, tuberculosis (TB) or a sexually transmitted disease (STD)
• People who received a blood transfusion prior to 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 he or she may have been exposed

A negative test for HIV antigen and/or HIV antibody usually indicates that a person does not have an HIV infection. A negative screening test means only that there is no evidence of disease at the time of the test, however. It is important for those who are at increased risk of HIV infection to have screening tests performed on a yearly basis to check for possible exposure to the virus.

HIV tests that detect only HIV antibody will not detect an HIV infection soon after exposure, during the window period before the development of antibodies. Most people produce detectable levels of antibody 3 to 12 weeks after exposure. If someone is screened with an HIV antibody test too soon, the result may be negative despite the fact that the person is infected (false negative). If an HIV antibody test is negative but suspicion of exposure remains high, then repeat testing using the HIV antigen/antibody blood test may be required.

If someone tests positive on both the initial screen and supplemental testing, then that person is considered to be infected with HIV.

The CDC recommends use of a new testing protocol to screen for and diagnose HIV infection. The following lists the steps and meaning of test results:

• Screen for HIV infection 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 of 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 infection cannot be cured, but early diagnosis allows for treatment with antiretroviral therapy (ART) that can help to suppress levels of virus in the body (viral load) and greatly improve long-term health. People typically take at least three drugs from two different classes in order to prevent or minimize virus replication and the emergence of drug-resistant strains. Combinations of three or more antiretroviral drugs are referred to as highly active antiretroviral therapy or HAART. (See Common Questions #2 for more details.)

There is currently no vaccine to protect against HIV, but avoiding high-risk activities such as having unprotected sex and sharing needles for injecting drugs can help to prevent its spread. Early diagnosis of HIV infection is important to prevent its transmission to others and to allow evaluation, monitoring, and early treatment of the affected person.

While there is no vaccine, the U.S. Centers for Disease Control and Prevention and the World Health Organization recommend that individuals without HIV infection but at high risk for it consider taking pre-exposure prophylaxis (PrEP), a daily pill to help prevent infection. For people taking PrEP consistently, the risk of HIV infection was up to 92% lower compared to those who didn't take it.

Treatment of HIV-infected mothers during pregnancy, precautions at birth, and avoiding breast-feeding can minimize the risk of passing the infection from mother to child. Giving the antiretroviral drug zidovudine intravenously during labor and delivery and also to the newborn twice a day by mouth for 6 weeks reduces the rate of transmission from 25-33% to about 1-2%. A combination of antiretroviral therapies is most effective at reducing the risk of HIV transmission to the baby.

Healthcare workers can protect themselves from HIV infection by following universal precautions, such as wearing gloves and avoiding needle sticks.