Testing is the only way to know your status when it comes to HIV. Many people who are infected with HIV don’t have symptoms. While testing may sound and feel scary—so does not knowing! These days, it’s pretty simple to get an HIV test at your doctor’s office or at a specialized HIV counseling and testing location. If you don’t know where to go for a test you can find updated testing sites at hivtest.cdc.gov.
When HIV enters the body, the immune system responds by producing antibodies—substances that recognize germs (such as viruses or bacteria) or cancer cells and mobilize the body to fight them. HIV antibody tests are designed to detect HIV antibodies rather than the HIV virus itself. In contrast, HIV antigen tests detect parts of the virus that trigger the body to produce antibodies. Polymerase chain reaction tests, typically called PCR tests for short, detect the genetic material of HIV itself.
The most common HIV tests detect HIV infection in a sample of blood. Some tests can also detect HIV in urine, fluid from the mouth, or a scraping from inside the cheek. Although it takes a few days to get the results of some HIV tests, rapid HIV tests can give results within an hour—sometimes as little as 10 to 20 minutes.
A positive (reactive) test result means that the test has found some evidence of HIV infection—HIV antibodies, antigens, or viral genetic material, depending on the type of test. A negative (nonreactive) test result means that no evidence of HIV infection has been found.
Occasionally, a person will get an indeterminate or inconclusive test result, which means that the test result is unclear. When a person gets an indeterminate result, the test should be repeated. If the second test also has an indeterminate result, than other types of tests may be performed to provide a clearer result.
Any positive HIV test result must be followed up by another test—called a confirmatory test—to confirm the positive result. It can take a few days to a few weeks to get the results of a confirmatory test.
Before considering different HIV tests in more detail, it is useful to define several important testing terms.
The window period of an HIV test is the amount of time from when a person is first infected with the virus to the appearance of detectable evidence of their HIV infection, such as HIV antibodies, antigens, or viral genetic material. Most people develop detectable HIV antibodies within 9 to 14 days after becoming infected. According to CDC, 97% of persons develop antibodies in the first 3 months after becoming infected with HIV, although in very rare cases, it may take up to 6 months to develop HIV antibodies.
The sensitivity of an HIV test refers to the test’s ability to detect what it is looking for, such as HIV antibodies. For example, a highly sensitive HIV antibody test will nearly always detect HIV antibodies if they are present. Such a test will have very few false-negative results.
The specificity of an HIV test refers to the test’s ability to eliminate findings that are similar to, but not, what it is looking for. For example, a highly specific HIV antibody test does a very good job distinguishing HIV antibodies from other substances that are somewhat similar to HIV antibodies. Such a test will have very few false-positive results.
The accuracy of an HIV test reflects both the test’s sensitivity and specificity. For example, a highly accurate HIV antibody test is one that has both high sensitivity and specificity in detecting HIV antibodies. Such a test will have very few false-positive and very few false-negative results.
As mentioned earlier, there are a number of different types of HIV tests. These include:
4th Generation HIV EIA Test
The 4th generation test can detect p24 antigen (an HIV protein), HIV-1 antibodies, and HIV-2 antibodies, but it cannot tell them apart. A positive result on the 4th generation test is followed up with a Multispot confirmatory test to detect whether antibodies to HIV-1 or HIV-2 are present. If no antibodies are detected, the NAAT test is used to detect HIV RNA. (You can read more about these tests below.) The 4th generation test has a window period of 2 weeks, compared to 2 to 8 weeks for a standard or rapid HIV antibody test. Testing sites in Massachusetts commonly use the 4th generation test because of its shorter window period.
A rapid HIV antibody test provides results within an hour, and sometimes substantially less. This test is done by a trained counselor through a finger prick or, in some places, an oral swab.
In the OraSure test, a swab is rubbed between the cheek and lower gum to collect a small sample of the tissue lining the inside of the mouth, which is then sent out for testing. It is recommended that people taking the OraSure test not eat, drink, or smoke within 20 minutes before the test is administered. At the lab, an EIA (and, if necessary, a confirmatory test) is performed on the sample to detect HIV antibodies. Results are typically available in 5 to 10 business days.
The U.S. Food and Drug Administration (FDA) has approved two home HIV antibody tests: the Home Access HIV-1 Test System and the OraQuick In-Home HIV Test. In the Home Access test, a person extracts a few drops of blood through a finger prick and collects the drops on a special card that is mailed in for testing. The OraQuick home test is designed to detect HIV in a sample of oral fluid. It can provide results in 20 minutes. A positive test result on these home HIV tests should be followed up with a confirmatory test.
Standard (Conventional) HIV Antibody Test
This is also often called ELISA (short for “enzyme-linked immunosorbant assay”) or EIA (short for “enzyme immunoassay”). In the standard HIV antibody test, a blood sample is drawn and then sent to a laboratory for testing. Results are usually available in 2 to 10 business days.
Western Blot Test
This antibody test may be used to confirm HIV infection in persons who have a positive result on an EIA. The Western blot can rule out false-positive results by distinguishing HIV antibodies from other antibodies that may react to the EIA. Results of Western blot tests are usually available in a few days to a few weeks. Antibody tests using an EIA, followed up with a Western blot, as needed, have about a 99.5% accuracy.
This antibody test may also be used to confirm HIV infection in persons who have a positive result on an EIA. It is more sensitive, more specific, faster, and less expensive than the Western blot. It can also distinguish between the two types of HIV—HIV-1 and HIV-2.
p24 Antigen Test
This test detects an HIV protein called p24. The p24 antigen can be detected 2 weeks after a person is infected.
Nucleic Acid Amplification Test (NAAT)
This is a highly sensitive PCR test that can detect HIV RNA—the virus’s genetic material. The NAAT test can detect the virus during acute HIV infection.
Other Types of HIV Tests
It is worth noting that some other types of HIV tests—viral load tests and resistance tests—are also available, but they are not used for diagnosing HIV infection. These tests are used instead for monitoring HIV disease and guiding HIV treatment decisions in persons who are already known to be infected with the virus.
According to the U.S. Preventive Services Task Force (USPSTF) 2013 HIV screening recommendations, healthcare providers should offer routine HIV screening to all adolescents and adults between 15 and 65 years old. The USPSTF also recommends HIV screening for all pregnant women, including women in labor who do not know their HIV status, as well as for teens younger than age 15 and adults older than 65 who are at increased risk for HIV infection.
Persons that the USPSTF considers to be at increased risk for HIV infection include: men who have sex with men (MSM); active injection drug users; persons who engage in unprotected vaginal or anal intercourse; persons with sexual partners who are HIV-infected, bisexual, or injection drug users; and persons who exchange sex for drugs or money.
Other persons considered at high risk include those who have acquired other sexually transmitted infections (STIs) or have requested testing for STIs. In addition, persons under age 15 or older than 65 with new sexual partners may also be considered to be at increased risk for infection and candidates for HIV screening.
The Massachusetts Department of Public Health also strongly endorses routine HIV screening in primary and urgent care “due to the significant individual and public health benefits associated with knowledge of HIV status and prompt linkage to medical care and preventive services.”
Routine HIV Screening
If a person is a candidate for routine HIV screening (see section above) and has had no potential exposure to HIV—such as through unprotected sex or needle-sharing—within the previous 8 weeks, then they can undergo testing without waiting.
HIV Testing Following a Recent Potential Exposure to HIV
If a person is being tested following a recent potential exposure to HIV, then they should typically wait until the window period for their particular HIV test has elapsed before undergoing testing. As described earlier, the window period is the time after a person is infected when there may not yet be enough evidence—such as HIV antibodies, antigens, or viral genetic material—for the test to produce an accurate result. So if for example, a person wanted to use a home antibody test, they would wait at least 8 weeks after their most recent potential HIV exposure to take the test, because it takes up to 8 weeks for most people to develop detectable HIV antibodies.
Repeat HIV Testing
In its 2013 HIV screening recommendations, the USPSTF notes there is not yet enough evidence to determine the best frequency for repeated HIV testing among persons who may have a higher-than-usual risk of HIV infection. In the meantime, “One reasonable approach would be one-time screening of adolescent and adult patients to identify persons who are already HIV-positive, with repeated screening of those who are known to be at risk for HIV infection, those who are actively engaged in risky behaviors, and those who live or receive medical care in a high-prevalence setting.” According to the Centers for Disease Control and Prevention (CDC), a high-prevalence setting is a geographic location or community in which at least 1% of the population is infected with HIV. These settings include STI clinics, correctional facilities, homeless shelters, tuberculosis clinics, clinics serving gay, bisexual, and other men who have sex with men (MSM), and adolescent health clinics with a high prevalence of STIs.
If you have specific questions about HIV testing or other HIV-related health issues, please contact AIDS Action’s Health Library at 617.450.1432 or visit the Health Library page.