HIV Test Accuracy: Chances of False Positives and Negatives

If you've recently been tested for HIV, or you're thinking about getting tested, you might have concerns about the possibility of receiving an incorrect test result.

With current methods of testing for HIV, incorrect diagnoses are very uncommon. But in rare cases, some people do receive a false-positive or false-negative result after being tested for HIV.

In general, it takes multiple tests to accurately diagnose HIV. A positive test result for HIV will require additional testing to confirm the result. In some cases, a negative test result for HIV may also require additional testing.

Read on to learn more about HIV test accuracy, how testing works, and the different testing options that are available.

In general, current HIV tests are highly accurate. HIV test accuracy depends on several factors, including:

• the type of test used
• how soon a person is tested after being exposed to HIV
• how a person's body responds to HIV

When a person first contracts HIV, the infection is considered acute. During the acute stage, it's difficult to detect. Over time, it becomes chronic and easier to diagnose with tests.

All HIV tests have a 'window period.' This is the period of time between when a person has been exposed to the virus and when a test can detect its presence in their body. If a person with HIV is tested before the window period has passed, it can produce false negative results.

HIV tests are more accurate if they're taken after the window period has passed. Some types of tests have shorter window periods than others. They can detect HIV sooner after exposure to the virus.

A false-positive result happens when a person who doesn't have HIV receives a positive result after being tested for the virus.

This can happen if laboratory staff mislabel or improperly handle a test sample. It can also happen if someone misinterprets the results of a test. Taking part in a recent HIV vaccine study or living with certain medical conditions might also lead to a false-positive test result.

If the first HIV test result is positive, a healthcare provider will order follow-up testing. This will help them learn if the first result was accurate or a false positive.

A false-negative result happens when a person who has HIV receives a negative result after being tested for the condition. False-negative results are less common than false-positive results, although both are rare.

A false-negative result can happen if a person gets tested too soon after contracting HIV. Tests for HIV are only accurate after a certain amount of time has passed since the person has been exposed to the virus. This window period varies from one type of test to another.

If a person gets tested for HIV within three months of being exposed to the virus and the result is negative, the U.S. Department of Health & Human Services recommends getting tested again in three months.

For antigen/antibody tests, retesting can be done sooner, about 45 days after suspected exposure to HIV. This will help determine if the first test result was accurate or a false negative.

Several types of tests are available for HIV. Each type of test checks for different signs of the virus. Some types of test can detect the virus sooner than others.

Antibody test

Most HIV tests are antibody tests. When the body is exposed to viruses or bacteria, the immune system produces antibodies. An HIV antibody test can detect HIV antibodies in blood or saliva.

If a person contracts HIV, it takes time for the body to produce enough antibodies to be detected by an antibody test. Most people develop detectable levels of antibodies within 3 to 12 weeks after contracting HIV, but it may take longer for some people.

Some HIV antibody tests are performed on blood drawn from a vein. To perform this type of antibody test, a healthcare professional may draw a sample of blood and send it to a lab for analysis. It may take several days for the results to become available.

Other HIV antibody tests are performed on blood collected through finger pricking or on saliva. Some of these tests have been designed for rapid use in a clinic or at home. The results of rapid antibody tests are typically available within 30 minutes. In general, tests from venous blood can detect HIV sooner than tests done from a finger prick or saliva.

Antigen/antibody test

HIV antigen/antibody tests are also known as combination tests or fourth generation tests. This type of test can detect proteins (or antigens) from HIV, as well as antibodies for HIV.

If a person contracts HIV, the virus will produce a protein known as p24 before the immune system produces antibodies. As a result, an antigen/antibody test can detect the virus before an antibody test can.

Most people develop detectable levels of p24 antigen 13 to 42 days (about 2 to 6 weeks) after contracting HIV. For some people, the window period may be longer.

To perform an antigen/antibody test, a healthcare professional may draw a sample of blood to send to a lab for testing. The results may take several days to come back.

Nucleic acid test (NAT)

An HIV nucleic acid test (NAT) is also known as an HIV RNA test. It can detect genetic material from the virus in blood.

In general, NAT can detect the virus before an antibody or antigen/antibody test can. Most people have detectable levels of the virus in their blood within 7 to 28 days after contracting HIV.

However, NAT is very expensive and is generally not used as a screening test for HIV. In most cases, a healthcare provider won't order it unless a person has already received a positive test result from an HIV antibody or antigen/antibody test, or if a person had a recent high-risk exposure or has symptoms of acute HIV infection.

For people taking pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP), these medications may reduce the accuracy of NAT. Let your healthcare provider know if you're using PrEP or PEP.

Healthcare providers may screen for HIV as part of a routine check-up, or people can request to be tested. The Centers for Disease Control and Prevention (CDC) recommends that everyone between the ages of 13 and 64 be tested at least once.

For those at heightened risk of contracting HIV, the CDC recommends being tested more often. For example, people who have multiple sexual partners have a higher risk of being exposed to HIV, and may choose more frequent testing, as often as every 3 months.

Your healthcare provider can talk to you about how often they recommend you be screened for HIV.

If the result from an initial HIV test is positive, a healthcare provider will order follow-up testing to learn if the result is accurate.

If the first test was conducted at home, a healthcare provider will draw a sample of blood to test in a lab. If the first test was done in a lab, follow-up testing may be conducted on the same blood sample at the lab.

If the second test result is positive, a healthcare provider can help explain the treatment options for HIV. Early diagnosis and treatment can help improve the long-term outlook and reduce the chances of developing complications from HIV.

In general, the chances of misdiagnosis for HIV are low. But for people who think they may have received a false-positive or false-negative test result for HIV, it's important to talk to a healthcare provider. They can help explain the test results and recommend next steps. For people at higher risk of contracting HIV, a healthcare provider can also recommend strategies for lowering the risk of infection.