HIV Test Types & Accuracy of Results of HIV Testing Kits

HIV (human immunodeficiency virus) testing facts

Every adult can benefit from undergoing HIV testing at least once.

• HIV testing is done to diagnose those who are newly infected, to identify previously unrecognized infections, and to relieve the minds of those who are not infected.
• New sexual partners should consider getting an HIV test before sex to inform themselves of how to keep from getting HIV. There is no vaccine, but there are other prevention methods.
• HIV can infect anyone who has sex, including those with heterosexual, bisexual, homosexual, or any variation of sexual contact, so an HIV test can benefit everyone.
• Every adult can benefit from being tested for HIV at least once. Many people never learn how they got HIV or from whom they got it. Some were monogamous, but their partner was having sex or was using an injected drug outside the relationship.
• There were about 40,000 new HIV infections in 2016. About 15% of those infected didn't know they had HIV.
• The sooner a person learns they have HIV, the sooner they can get a referral for treatment and live a fairly normal life. Avoiding this knowledge only shortens life and well-being and puts other people at risk in the long run.
• HIV testing should be a routine part of medical practice. Most states no longer require formal signed consent for HIV testing.
• It is critical that pregnant women be tested because medications are very effective in reducing transmission of HIV from mother to baby.
• HIV testing is usually a two-step process. The first step is to test for HIV-specific proteins (antibodies or antigens) in blood or saliva. If the test is positive, a second test called a Western blot is done to ensure that the first result was correct.
• If both tests are positive, the chances are >99% that the patient is infected with HIV.
• HIV tests may miss some infections, resulting in false-negative tests. This often occurs soon after infection when antibodies have not yet developed or are just starting to form and are at a level too low to be detected (within about four weeks of infection).
• There are free HIV testing locations in every state.

Types of HIV Tests

The body makes antibodies to try to fight HIV, although the antibodies cannot eradicate the virus. Antibody testing is often done in two parts. First a sensitive screening test is performed on the blood. If the screening test is positive, a second test is done to confirm that HIV antibodies are present. The types of tests have varied over the years. At first, the screening test used an enzyme-linked immunoassay (ELISA) with confirmatory testing by Western blot. This strategy did not test for HIV-2, sometimes misclassified HIV-2 infection, missed very early infections where antibody had not yet been produced, and sometimes produced indeterminate results.

Read more about different types of HIV tests »

What is the human immunodeficiency virus (HIV)?

HIV is short for human immunodeficiency virus. This virus causes the acquired immunodeficiency syndrome or AIDS. HIV is a complicated virus. It reproduces primarily in specialized cells of the body's immune system called CD4 lymphocytes. During HIV replication, the CD4 cells are destroyed. As more and more cells die, the body loses the ability to fight many infections. If the number of CD4 cells in the bloodstream falls below 200 per cubic millimeter, or if some other special health conditions occur, the person is defined as having AIDS. These special health conditions include infections and cancers that take advantage of the suppressed immune system. Regardless of the CD4 count, people with HIV infection carry the virus and can spread it to others through unprotected sex or contact with blood or some other body fluids.

Undiagnosed HIV infection is responsible for continued transmission, even in up to one-third of transmissions in the U.S. Of 1.2 million estimated cases in individuals over 13 years of age as of December 2012 (U.S. Centers for Disease Control and Prevention [CDC] estimates), about 15% of them are likely to be unaware of their infection. Thus, an HIV test is important to diagnose those who are newly infected, to identify previously unrecognized infections, and to relieve the minds of those who are not infected. HIV testing also reduces the risk of transmission during pregnancy, blood transfusions, and tissue transplantation.

When should a person undergo HIV testing?

The CDC recommends a routine HIV test for adolescent and adult patients aged 13 to 64 in all health care settings, of all women during pregnancy, and the newborns of HIV-positive women. Thus, HIV testing is now part of routine medical practice, similar to tests that screen for other diseases. In 2006, the CDC recommended eliminating written HIV-specific consent for testing in health care facilities in order to reduce unwarranted stigma and encourage screening. HIV test consent is included with general medical consent forms, and patients are informed that HIV testing will be performed as routine unless they decline (opt-out screening). As of January 2015, all states but Nebraska had adopted routine opt-out HIV testing.

People who are at high risk for acquiring HIV should undergo annual HIV testing. Sometimes, health care professionals request or require testing as part of evaluation and treatment for other conditions, such as women undergoing treatment with assisted reproductive technologies for infertility or treatment of viral hepatitis. There is increasing concern that not enough people are being tested. Events such as National HIV Testing Day have raised awareness and increased participation in testing.

In some cases, HIV testing may be required by law. This occurs for blood used for transfusions, organ donors, and military personnel. States may select additional populations for mandatory testing, such as prisoners or newborns.

What are the different types of HIV testing?

There are three main types of HIV tests: antibody tests, RNA (viral load) tests, and a combination test that detects both antibodies and viral protein called p24 (antibody-antigen test, or HIV Ab-Ag test). All tests are designed to detect HIV-1, which is the type of HIV in the United States. Some antibody tests and the combination test can also detect HIV-2 infections, which are usually limited to West Africa. No test is perfect; tests may be falsely positive or falsely negative or impossible to interpret (indeterminate, see below).

Positive test results are reportable to the health department in all 50 states and include the patient's name. This information is then reported to the CDC (without names) so that the epidemiology and infection spread rates can be monitored. The names sent to the state remain confidential and will not be reported to employers, family members, or other such people. Some states allow anonymous testing in which the patient's name is not recorded.

HIV antibody tests: HIV possesses many unique proteins on its surface and inside the virus itself. When someone is infected with HIV, their body produces proteins designed to tag the virus for elimination by the immune system. These proteins are called antibodies, and they are directed against the unique proteins of HIV. Unfortunately, these HIV antibodies do not eliminate the virus, but their presence serves as a marker to show that someone is infected with HIV. HIV antibody tests are the most commonly used tests to determine if someone has HIV.

Antibody testing is usually done on a blood sample, often using an enzyme-linked assay called an ELISA or EIA. In this test, a person's serum is allowed to react with virus proteins that have been produced in the laboratory. If the person has been infected with HIV, the antibodies in the serum will bind to the HIV proteins, and the extent of this binding can be measured. Negative EIA results are usually available in a day or so.

There are some rapid HIV testing kits on the market that can be used in a health care professional's office or other points of care. Most of these kits still require blood to be drawn, although it can be done using a simple finger stick in some cases.

Home-testing is also possible and may be more convenient for some individuals. The Home Access HIV-1 Test System was the first home testing kit approved by the U.S. FDA in 1996. The test involves pricking a finger, adding a drop of blood to a test strip, and mailing the sample to a laboratory. Users received results by phone using an anonymous code. Home Access Corporation discontinued sales in December 2018. The OraQuick In-Home HIV test kit was FDA-approved in 2013. As of 2019, it is the only home HIV test that is FDA-approved in the U.S. It is sold online or in drug stores. The advantage of OraQuick is that it tests for antibodies in saliva (oral fluid) instead of blood, and users can read the results within 20-40 minutes at home. Users obtain their oral fluid by swabbing their gums.

Both blood and saliva testing are 99.9% accurate at predicting that there is no HIV in the sample. But because levels of HIV are higher in blood than saliva, the saliva test is slightly more likely to miss HIV in saliva (91.7% accurate) than in blood (99.7% accurate). Thus, OraQuick might miss HIV virus (false-negative test) in about one in 10 people, especially if performed soon after infection. A person who strongly suspects a recent HIV infection should get a more sensitive blood test performed by a lab. (See HIV antibody-antigen [Ab-Ag] test.)

Because there is a small chance that a person's antibodies will falsely attach to the non-HIV proteins during the test, a second, more specific test is done on all initially positive antibody tests. This second test is called the Western blot test. In this test, the HIV proteins are separated by size and electric charge and the person's serum is layered on the test strip. If the test is positive, a series of bands are detected that indicate specific binding of the person's antibody to specific HIV virus proteins. This test is only done to follow up an initially positive screening test. It is not as helpful when performed on its own.

HIV RNA tests: The HIV RNA is different than all human RNA, and tests have been developed to detect HIV RNA in a person's blood. Because this test can be used to estimate the amount of circulating HIV in the blood, it is often referred to as an HIV viral load. This uses a type of test called a polymerase chain reaction (PCR). These tests are important for newborn screening of HIV-positive mothers since maternal antibody may cross the placenta and be present in the newborn. These tests may also be helpful in detecting HIV infection in the first four weeks following exposure, before antibodies have had time to develop. However, they are costly and are not routinely used to screen for infection.

HIV antibody-antigen (Ab-Ag) test: The HIV Ab-Ag test detects antibodies directed against HIV-1 or HIV-2, as well as a protein called p24, which forms part of the core of the virus (an antigen of the virus). This is important because it takes weeks for antibodies to form after the initial infection, even though the virus (and the p24 protein) is present in the blood. Thus, Ab-Ag testing may allow for earlier detection of HIV infections. Preliminary studies suggest that diagnosis could be made an average of one week earlier using the Ab-Ag test, compared to antibody testing alone. The test uses a reaction known as "chemiluminescence" to detect antibodies and p24 protein antigen. In other words, if either the antibody or the antigen is present, the test reaction emits light that registers on a detector. There is only one currently approved antibody-antigen test, the Architect HIV Ag/Ab Combo assay. If this test is positive, it is recommended it be repeated. Tests that remain positive are confirmed with Western blot as described above.

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How long does it take to get results back from an HIV test?

Results from antibody tests that are sent to a laboratory usually take one to three days to return, but this varies depending on the test, the laboratory, and whether it is a home test kit. If the test is positive, results may be delayed while the laboratory does a Western blot to be sure that HIV antibody is present. Western blot tests take only one day to perform, but some laboratories may not run the test every day. Results from rapid tests done in the health care professional's office or at other points of care are usually available in 15-20 minutes. If the rapid test is positive, it is still necessary to send blood to a laboratory for a Western blot to confirm that the rapid test result is correct. Home tests are mailed to a laboratory, and results return in one to two weeks, depending on the manufacturer's instructions. RNA testing results usually take a few days to a week depending on the lab.

Is counseling offered with HIV testing?

Each state establishes requirements for HIV counseling. Most states have supported an opt out testing program in which people are recommended to have HIV testing but may opt out if they choose. Such programs should include counseling about HIV, including prevention of infection, the meaning of the HIV test, and the need for appropriate follow-up. Opt-out programs have resulted in earlier diagnosis of many people with HIV.

HIV testing may sometimes be done if a health care worker or first responder (police officer, fireman, emergency medical technician, etc.) has significant exposure to the blood or body fluid from an identifiable person. In this situation, testing without consent is available in most states. All blood and plasma donors, some prisoners, and some military personnel are tested for HIV.

What is HIV?
See Answer

How accurate is an HIV test? What is the window period for an HIV test?

The current testing protocols are highly accurate but not perfect. The probability of a false result on the test depends on the test and on the person's risk factors for getting infected. The lower the risk of getting HIV, the higher the probability of a false- positive result.

Falsely negative tests occur in people who are truly infected with HIV but have negative tests. Among 1,000 people who are truly infected, rapid tests will be falsely negative in zero to six people, depending on the test. Negative antibody tests in people infected with HIV may occur because antibody concentrations are low or because antibodies have not yet developed. On average, antibodies take about four weeks to reach detectable levels after initial infection, and falsely negative tests may occur during this so-called HIV window period. Individuals with negative tests and who had high risk for HIV exposure should be retested in two to three months.

Falsely positive tests occur when uninfected people have positive results. Among 1,000 people who do not have disease, rapid tests will be falsely positive in zero to nine people, depending on the test. This is the main reason for not relying on a single positive test for diagnosis. As discussed above, all positive initial tests must be confirmed with a follow-up test (Western blot). When both tests are positive, the likelihood of a person being HIV infected is >99%. Sometimes, the Western blot may be indeterminate, meaning that it is neither positive nor negative. In these cases, the tests are usually repeated at a later date or an RNA test is done.

Is HIV testing necessary for pregnant women?

HIV testing is critically important for pregnant women. HIV testing is recommended at the beginning of each pregnancy during prenatal care. If any HIV risk factors are present or there is a high incidence of HIV in the population, testing should be repeated in the third trimester. There have been enormous advances in the treatment of HIV-infected pregnant women. With proper management, the probability of transmitting the virus to the fetus is less than 2%. Without proper management, the risk of transmission is as high as 33%. Because undiagnosed HIV is so common, it is necessary to test all pregnant women. It is strongly recommended that all children born to women with HIV also be tested.

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Where can
people find free HIV testing locations?

The CDC maintains a list of HIV testing locations for people who want to find out whether they have contracted the virus. This National HIV and STD Testing Resource can be accessed at http://www.hivtest.org. This site includes the ability to search for free testing locations as well as locations that provide rapid tests. Some clinics only provide HIV testing. However, sexually transmitted diseases (STDs) clinics routinely provide HIV testing along with testing for diseases like chlamydia, gonorrhea, syphilis, and herpes.

Where can people find more information about HIV testing?

There are several resources for people interested about the facts of HIV testing.

• The national HIV, STD, and hepatitis testing site Get Tested helps visitors find free, fast, and confidential testing.

The CDC web site is also an excellent source of information: https://www.cdc.gov/hiv/basics/testing.html.