HIV Antibody and HIV Antigen (p24)
Also known as:HIV Screening Tests; AIDS Test; AIDS Screen; HIV Serology; p24 Antigen
Formal name:HIV-1 and HIV-2 Antibody and Antigen Evaluation
Related tests:CD4 Count; HIV Viral Load; HIV Genotypic Resistance Testing
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Why Get Tested?
To determine if you are infected with human immunodeficiency virus (HIV)
When to Get Tested?
At least once, when you are 13 to 64 years old; when you think you may have been exposed to the virus; before becoming pregnant or when pregnant; once a year if you are at increased risk of being exposed to the virus
A blood sample collected from a vein in your arm or from a fingerstick; there are also HIV antibody tests available that can be performed on oral fluid.
Test Preparation Needed?
How is it used?
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.
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.
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.
When is it ordered?
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
What does the test result mean?
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.
Is there anything else I should know?
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.
What is being tested?
human immunodeficiency virus (HIV) is the cause of AIDS (acquired immunodeficiency syndrome). HIV screening tests detect the HIV antigen (p24) and/or HIV antibodies produced in response to an HIV infection in the blood. Some tests detect HIV antibody in oral fluid.
When a person becomes infected with HIV, through exposure to the blood or body fluids of an infected individual or a contaminated needle, for example, the virus begins to replicate itself, producing a large number of copies. During the first few weeks of infection, the amount of virus (viral load) and the p24 antigen level in the blood can be quite high.
About 2-8 weeks after exposure to the virus, the immune system responds by producing antibodies directed against the virus that can be detected in the blood. As the initial infection resolves and the level of HIV antibody increases, both virus and p24 antigen levels decrease in the blood.
An HIV infection may initially cause no symptoms or cause flu-like symptoms that resolve after a week or two. The only way to determine whether a person has been infected is through HIV testing.
If HIV is not detected early and treated, it may become a simmering infection that may cause few symptoms for a decade or more. If the infection is still not treated, eventually symptoms of AIDS emerge and begin to progressively worsen. Over time and without treatment, HIV destroys the immune system and leaves a person's body vulnerable to debilitating infections. (You can read more about this in the article on HIV Infection and AIDS.)
Detecting and diagnosing HIV early in the course of infection is important because:
- It allows for early treatment that slows progression to AIDS.
- An individual can learn of their status and modify behavior so as to prevent the spread of disease.
- A pregnant woman can undergo treatment that would help prevent passing the disease to her child.
There are two types of HIV, 1 and 2. HIV-1 is the most common type found in the United States, while HIV-2 has a higher prevalence in parts of Africa.
A few different testing options are available for HIV screening:
- Combination HIV antibody and HIV antigen test–the recommended screening test for HIV; it is available only as a blood test.
- 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.
Regardless of the type of screening test used, a positive result requires follow up with supplemental testing to establish a diagnosis of HIV. (For more details, see "The Test" section.)
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm or from a fingerstick. There also are tests available that can be performed on oral samples. An oral sample is obtained by using a special small, spatula-like device with a flat pad on the end. The flat pad is placed above the teeth against the outer gum and is swabbed completely once around the outer part of the upper and lower gums.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
- What are the symptoms of HIV infection?
Symptoms of the initial HIV infection can mimic those of influenza and other viral infections. The only reliable way to tell if someone is infected is to get tested. Many people with HIV do not experience symptoms for years after the initial infection or have symptoms that are very similar to symptoms of other illnesses. For more, see this CDC web page: HIV/AIDS Basics.
- What are the treatments for HIV/AIDS?
Currently, there is no cure for HIV infection or AIDS. However, there are therapies that can improve immune function, reduce the risk and/or frequency of opportunistic infections, and prolong life. The U.S. Department of Health and Human Services (DHHS) Panel on Antiretroviral Guidelines for Adults and Adolescents as well as the World Health Organization recommend that all individuals diagnosed with HIV infection receive treatment as soon as possible, including pregnant women. With advances in treatment, individuals with HIV infection are living longer, healthier lives. 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. Read the Treatment section of the article on HIV Infection and AIDS for additional details.
- Should I tell anyone else of my test results?
Yes. If you test positive for HIV infection, it is important that you tell your healthcare practitioners as well as all current and future sex partners and/or anyone with whom you share needles. Counseling services are often available from the clinic that performed the test or from your healthcare provider that will help you to inform the people who need to know.
- How confidential are HIV test results?
Your HIV status, like other medical conditions and test results, is protected by the HIPAA Privacy Rule and cannot be shared with friends, family, or employers without your written permission. Your HIV status may be shared with your healthcare providers who have a "need to know" in order to treat you. Also, in order to determine the incidence of HIV and to provide appropriate prevention and care services, all new cases of HIV are reported to state and local health departments.
Certain testing centers provide either anonymous (your name is never given) or confidential (your name is given but kept private) HIV testing and counseling. The FDA has approved one home testing device that allows you to remain anonymous and to get confidential results. You can also contact your state, county, or city health department to find out where testing may be available.
- Can you use the HIV antibody test to detect HIV in newborns?
No. Because maternal antibodies are transferred from mother to baby and stay in the newborn's system for 6-12 months, a different test must be used. A test that detects the genetic material, either an HIV RNA or HIV DNA test, is required.
© 2017 American Association for Clinical Chemistry, republished from Lab Tests Online.*
Descriptions of clinical laboratory tests were originally prepared for use on Lab Tests Online, an award-winning patient education website on clinical laboratory testing. Lab Tests Online is produced by the American Association for Clinical Chemistry (AACC), a global scientific and medical professional organization dedicated to clinical laboratory science and its application to healthcare. The Lab Tests Online website is developed in collaboration with other laboratory professional societies and is funded in part through corporate sponsorships.