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Basic Questions and Answers About HIV Testing

July 7, 2016

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What Kinds of Tests Are Available, and How Do They Work?

There are three broad types of tests available: antibody tests, combination or fourth-generation tests, and nucleic acid tests (NAT). HIV tests may be performed on blood, oral fluid, or urine.

  1. Most HIV tests, including most rapid tests and home tests, are antibody tests. Antibodies are produced by your immune system when you're exposed to viruses like HIV or bacteria. HIV antibody tests look for these antibodies to HIV in your blood or oral fluid. In general, antibody tests that use blood can detect HIV slightly sooner after infection than tests done with oral fluid.

    It can take 3 to 12 weeks (21-84 days) for an HIV-positive person's body to make enough antibodies for an antibody test to detect HIV infection. This is called the window period. Approximately 97% of people will develop detectable antibodies during this window period. If you get a negative HIV antibody test result during the window period, you should be re-tested 3 months after your possible exposure to HIV.
    • With a rapid antibody screening test, results are ready in 30 minutes or less.
    • The OraQuick HIV Test, which involves taking an oral swab, provides fast results. You have to swab your mouth for an oral fluid sample and use a kit to test it. Results are available in 20 minutes. The manufacturer provides confidential counseling and referral to follow-up testing sites. Because the level of antibody in oral fluid is lower than it is in blood, blood tests find infection sooner after exposure than oral fluid tests. These tests are available for purchase in stores and online. They may be used at home, or they may be used for testing in some community and clinic testing programs.
    • The Home Access HIV-1 Test System is a home collection kit, which involves pricking your finger to collect a blood sample, sending the sample by mail to a licensed laboratory, and then calling in for results as early as the next business day. This test is anonymous. The manufacturer provides confidential counseling and referral to treatment.

If you use any type of antibody test and have a positive result, you will need to take a follow-up test to confirm your results. If your first test is a rapid home test and it's positive, you will be sent to a health care provider to get follow-up testing. If your first test is done in a testing lab and it's positive, the lab will conduct the follow-up testing, usually on the same blood sample as the first test.

  1. A combination, or fourth-generation, test looks for both HIV antibodies and antigens. Antigens are foreign substances that cause your immune system to activate. The antigen is part of the virus itself and is present during acute HIV infection (the phase of infection right after people are infected but before they develop antibodies to HIV). If you're infected with HIV, an antigen called p24 is produced even before antibodies develop. Combination screening tests are now recommended for testing done in labs and are becoming more common in the United States. There is now a rapid combination test available.

    It can take 2 to 6 weeks (13 to 42 days) for a person's body to make enough antigens and antibodies for a combination, or fourth-generation, test to detect HIV. This is called the window period. If you get a negative combination test result during the window period, you should be retested 3 months after your possible exposure.
  1. A nucleic acid test (NAT) looks for HIV in the blood. It looks for the virus and not the antibodies to the virus. The test can give either a positive/negative result or an actual amount of virus present in the blood (known as a viral load test). This test is very expensive and not routinely used for screening individuals unless they recently had a high-risk exposure or a possible exposure with early symptoms of HIV infection.

    It can take 7 to 28 days for a NAT to detect HIV. Nucleic acid testing is usually considered accurate during the early stages of infection. However, it is best to get an antibody or combination test at the same time to help the doctor interpret the negative NAT. This is because a small number of people naturally decrease the amount of virus in their blood over time, which can lead to an inaccurate negative NAT result. Taking pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP) may also reduce the accuracy of NAT if you have HIV.

Talk to your health care provider to see what type of HIV test is right for you.

After you get tested, it's important for you to find out the result of your test so that you can talk to your health care provider about treatment options if you're HIV-positive. If you're HIV-negative, continue to take actions to prevent HIV, like using condoms the right way every time you have sex and taking medicines to prevent HIV if you're at high risk.

Learn more about how to protect yourself, and get information tailored to meet your needs from CDC's HIV Risk Reduction Tool (BETA).


How Soon After an Exposure to HIV Can an HIV Test Detect if I Am Infected?

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No HIV test can detect HIV immediately after infection. If you think you've been exposed to HIV, talk to your health care provider as soon as possible.

The time between when a person gets HIV and when a test can accurately detect it is called the window period. The window period varies from person to person and also depends upon the type of HIV test.

  • Most HIV tests are antibody tests. Antibodies are produced by your immune system when you're exposed to viruses like HIV or bacteria. HIV antibody tests look for these antibodies to HIV in your blood or oral fluid.
    The soonest an antibody test will detect infection is 3 weeks. Most (approximately 97%), but not all, people will develop detectable antibodies within 3 to 12 weeks (21 to 84 days) of infection.
  • A combination, or fourth-generation, test looks for both HIV antibodies and antigens. Antigens are foreign substances that cause your immune system to activate. The antigen is part of the virus itself and is present during acute HIV infection (the phase of infection right after people are infected but before they develop antibodies to HIV).
    Most, but not all people, will make enough antigens and antibodies for fourth-generation or combination tests to accurately detect infection 2 to 6 weeks (13 to 42 days) after infection.
  • A nucleic acid test (NAT) looks for HIV in the blood. It looks for the virus and not the antibodies to the virus. This test is very expensive and not routinely used for screening individuals unless they recently had a high-risk exposure or a possible exposure with early symptoms of HIV infection.
    Most, but not all people, will have enough HIV in their blood for a nucleic acid test to detect infection 1 to 4 weeks (7 to 28 days) after infection.

Ask your health care provider about the window period for the test you're taking. If you're using a home test, you can get that information from the materials included in the test's package. If you get an HIV test within 3 months after a potential HIV exposure and the result is negative, get tested again in 3 more months to be sure.

If you learned you were HIV-negative the last time you were tested, you can only be sure you're still negative if you haven't had a potential HIV exposure since your last test. If you're sexually active, continue to take actions to prevent HIV, like using condoms the right way every time you have sex and taking medicines to prevent HIV if you're at high risk. Learn the right way to use a male condom.

Learn more about how to protect yourself, and get information tailored to meet your needs from CDC's HIV Risk Reduction Tool (BETA).


Where Can I Get Tested?

You can ask your health care provider for an HIV test. Many medical clinics, substance abuse programs, community health centers, and hospitals offer them too. You can also find a testing site near you by

  • calling 1-800-CDC-INFO (232-4636),
  • visiting gettested.cdc.gov, or
  • texting your ZIP code to KNOW IT (566948).

You can also buy a home testing kit at a pharmacy or online.

Learn more about how to protect yourself, and get information tailored to meet your needs from CDC's HIV Risk Reduction Tool (BETA).


What Should I Expect When I Go in for an HIV Test?

If you take a test in a health care setting, when it's time to take the test, a health care provider will take your sample (blood or oral fluid), and you may be able to wait for the results if it's a rapid HIV test. If the test comes back negative, and you haven't had an exposure for 3 months, you can be confident you're not infected with HIV.

If your HIV test result is positive, you may need to get a follow-up test to be sure you have HIV.

Your health care provider or counselor may talk with you about your risk factors, answer questions about your general health, and discuss next steps with you, especially if your result is positive.

See Will other people know my test result?

Learn more about how to protect yourself, and get information tailored to meet your needs from CDC's HIV Risk Reduction Tool (BETA).

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This article was provided by U.S. Centers for Disease Control and Prevention. Visit the CDC's website to find out more about their activities, publications and services.
 

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