What are HIV and AIDS?

People often believe that if they have had a physical exam, the doctor has checked for HIV. This is often not true. HIV testing is mostly not done unless you have a certain type of medical problem, are pregnant, or ask to be tested.

This may be changing, however slowly. In 2006 the Centers for Disease Control (CDC) put out guidelines for health care providers to start routinely testing their patients aged 13 to 64 for HIV infection. The guideline no longer requires doctors to counsel patients before the test. Instead, the doctor lets patients know that HIV testing will be done along with routine blood tests. Different states may require doctors to give you more information or get you to sign a form before you can be tested for HIV. Because requirements vary from state to state and many doctors don't follow the CDC guidelines, you may want to ask your primary care doctor whether his or her office performs routine HIV testing.

HIV testing is still more likely to be done if you have symptoms that look like AIDS, or if you are pregnant. The doctor or nurse should let you know that the test will be included along with other blood work.

Most of the time, a person who is tested will know it. There are a few cases where people may not be told they are getting an HIV test. For instance, prisoners entering the penal system, people entering military service, and some people who apply for individual health or life insurance policies may be tested without knowing about it.

If you have any doubt about your HIV status, talk with your doctor or visit a health department clinic where testing is offered. To have the HIV test done without giving your name and address (anonymous testing), you can buy a home collection kit at the drugstore or go to a special anonymous testing site. Some state health departments and a few other health sites offer anonymous HIV tests, but call first to be sure.

Because HIV infection often has no symptoms for years, a person can have HIV for a long time and not know about it. People who do not seek HIV testing may not learn that they have HIV until they develop early symptoms or even AIDS. But with testing, HIV infection can be detected easily and treatment started before a person gets seriously ill.

Tests to detect HIV infection

You can get HIV testing in many settings (hospitals, neighborhood clinics, health department clinics, and doctors' offices). And there are HIV test kits that can be used at home. See the section, "Types of HIV tests" below.

Understanding HIV antibody tests and the test results

Depending on where the test is done, it can take a few days to 2 weeks to get the results. Most testing locations prefer that you return in person for your test results. Telephone results are usually given only for home kit testing, after the blood sample you mailed in is tested.

HIV is most often found using a sequence of screening tests that look for antibodies to HIV in the blood. When the body is infected with HIV, it starts to make antibodies (immune system proteins) against the virus. Although these antibodies can't get rid of HIV, they can be found in the blood and some other body fluids within several weeks of infection. HIV screening tests look for these antibodies, not the virus itself. The virus is harder to detect than the antibodies.

In the United States, the first test used for most HIV tests is called the HIV enzyme-linked immunosorbent assay (ELISA or EIA). This is a very sensitive test that can find antibodies to HIV in the blood. Because it is so sensitive, it sometimes finds "look-alike" substances that are not HIV antibodies, which can give a false positive result in some people who don't have HIV. This is why a second test is used to confirm a positive ELISA test.

If the ELISA test finds nothing, the HIV test result is reported as negative. This means that no trace of antibodies to HIV could be found. No further tests on this sample are needed, although a second or third test may be needed later for those who were recently exposed (see below).

If the ELISA result is positive, the same sample is checked by using a more specific test, usually the Western blot test. If this test result is negative, the HIV test result is reported as negative, since antibodies to HIV were not found on this more specific test.

If both test results are positive, the person is considered to be infected with HIV.

In rare cases, the Western blot test results come back not as positive or negative, but "indeterminate" (the test cannot tell if it is positive or negative). Sometimes, an indeterminate test means that the person has recently been infected with HIV but does not yet have enough antibodies to be sure. Because of this, people who have indeterminate results usually repeat the HIV test a few weeks later after antibodies would have had time to build up. Many times, the antibody test will be positive or negative on the repeat test. In a few people the test remains indeterminate. Most of the time, a test that stays indeterminate is caused by an unrelated or unknown condition.

The possible test results are outlined in the table below.

Table 1. How ELISA and Western Blot tests determine final HIV antibody test results

It's important to know that a negative test result does not mean for sure that a person does not have HIV. It takes several weeks after becoming infected with the virus before enough antibodies are produced to be detected. Tests done before this may be negative, even if a person is infected. To be sure that the person doesn't have the virus, the person should be tested 3 to 6 months after the most recent suspected exposure.

Ways to get antibody testing for HIV

Blood test: This is the most common way to test for HIV. A blood sample is taken in a doctor's office, clinic, hospital, etc., and is tested with an ELISA test. If this is positive, a Western blot test is done on the same sample. Results usually take a few days to 2 weeks.

Oral fluid test: This test looks for HIV antibodies in fluid taken through the mouth. For this test, a special test pad is placed between the cheek and the gum for about 2 minutes. The pad absorbs fluid from the bloodstream through the membranes of the cheek. The sample is then tested for antibodies, much like a blood sample. It may take a few days to 2 weeks to get these results.

Urine test: HIV antibodies are also present in small amounts in the urine. So an ELISA-type test and Western blot test have been developed to detect HIV in the urine. Getting this test result also takes a few days to 2 weeks, depending on the lab.

Quick tests: The tests are only used by trained staff. Depending on the test, antibodies to HIV in blood or oral fluid can be found within 10 to 20 minutes. Positive results must be confirmed with a separate Western blot test, which can take several days after the second blood sample is taken. Negative results require no further testing (unless possible HIV exposure was recent and antibodies haven't had time to develop, in which case one or more blood samples will be needed later.) The quick test is only offered in certain settings where the second test can be done for people with positive results. This test can also be done on fluid taken from the mouth.

Home testing: There are a few HIV home test kits being advertised on the market today, but as of 2010 only the Home Access^® HIV-1 Test System is FDA approved and legally marketed in the United States. The Home Access test kit has many pieces, including materials to collect a blood sample, a mailing envelope to send it to the lab, and information about HIV and the test.

The FDA-approved system uses a simple finger-stick process to collect blood at home. Blood is placed on a special paper in the kit and allowed to dry. The paper with the dried blood is mailed to a lab along with a personal identification number (PIN). It is tested in a certified medical lab the same way as samples taken in a doctor's office. The person who mailed the test calls the toll-free number from the kit and gives the PIN number to get their results.

You may also see home test kits that are not FDA approved. These are advertised and sold in newspapers, magazines, and on the Internet. Some falsely claim that the tests are FDA approved or "manufactured in an FDA-approved facility." The advertisers of these unapproved HIV home test kits make them sound as simple as a home pregnancy test. For instance, they often claim that the presence of a visible sign, such as a red dot, within 5 to 15 minutes of taking the test shows a positive result for HIV infection. The samples are not sent to a lab for professional testing. In the past, samples of these tests have been shown to give false negative results. Although unapproved home tests may seem simpler and faster, they can give unreliable results and should be avoided.

If you would like to know if a certain HIV home test has been approved by the FDA, you can check their information sheet on the Internet at www.fda.gov/BiologicsBloodVaccines/SafetyAvailability/HIVHomeTestKits/ucm126460.htm, or you can call the FDA at 1-800-835-4709. You can also email them at ocod@fda.hhs.gov.

Testing newborns: The usual HIV antibody tests are not helpful in newborns whose mothers are known to have HIV. Babies are born with their mothers' antibodies, which stay with them for several months. This means that any baby born to an HIV-infected mother will test positive for antibodies to HIV. This can be very confusing, because it means an infant's blood can test HIV-positive when the baby is not actually infected. Rather than wait a year or more for all of the mother's antibodies to go away, tests that detect the virus itself are used to find out whether an infant got HIV from the mother. Even with these special tests, it may take a few weeks to be certain of the infant's true HIV status. It is important to find out soon, because newborns with HIV can be treated early to help keep them from getting sick.

Other HIV lab tests

Other types of lab tests may be used to look for HIV. In general, these tests are used only in certain situations, such as when testing newborns (see section above).

• Viral culture methods: Viral culture tests involve growing live HIV from infected cells. Viral culture was the first method ever used to detect the HIV virus itself instead of the antibody response, but it is slow and expensive.
• Polymerase chain reaction (PCR): PCR is a very sensitive test by which the genes of a cell are increased many times to detect HIV viral DNA or RNA. This allows the PCR to look for the virus rather than the antibodies.
• Indirect immunofluorescence assay (IFA): For this test, a fluorescent dye is used to detect HIV antibodies. It is sometimes used instead of a Western Blot test to confirm a positive ELISA.

AIDS and immune function

A diagnosis of AIDS is made when a person tests positive for HIV and their immune function is no longer normal. Poor immune function can be shown by certain opportunistic infections or cancers (see "The definition of AIDS" section). Also, lab tests that count T helper cells can show poor immune function (see below). People with HIV are tested every few months to see how high their T helper cell counts are; the results help the doctor decide when HIV treatment is needed. T helper cells are also counted regularly after anti-HIV drugs are started, to be sure that the drugs are working as they should.

If a person has been diagnosed with AIDS based on a positive HIV test and a low T helper cell count, he or she is still considered to have AIDS even if treatment later raises the CD4 count. If a person is diagnosed with AIDS based on a positive HIV test and having an AIDS-defining illness, he or she still has AIDS even if the AIDS-defining illness is cured.

The immune profile, including CD4 or T-cell counts

An immune profile, or immune panel, is a set of blood tests that can be used to measure the loss of immunity and help decide on HIV treatment. This profile usually includes counts of different types of cells in the blood (a complete blood cell count, or CBC). More important, it counts the number and percentage of helper T cells or T helper cells. These are also called CD4 cells, CD4+ T lymphocytes, or simply T cells.

The number of helper T cells reflects the newly diagnosed patient's stage of infection and is used to find out how much damage HIV has done to the immune system. This lab test counts the number of helper T cells in each cubic millimeter (mm³) of blood. A normal helper T cell count in a healthy, HIV-negative adult can vary but is usually between 500 and 1,500 cells/mm³. (In babies and very young children the normal counts are much higher.) HIV infection lowers the T helper cell count over time. When a person gets anti-HIV drugs, this test is used to find out if the drugs are boosting the immune system. People begin to get opportunistic infections or cancers more often as the count drops lower.

Viral load test (HIV-1 RNA)

This test measures how much HIV (the viral load) is found in a small volume of blood. The viral load test is done after a patient has tested positive for HIV. Unlike the immune panel, this test does not show how much damage has been done to the immune system, so it cannot show whether a person has AIDS. But it can give the doctor an idea how quickly the HIV infection is likely to progress (get worse). A high viral load, even in a person with good T helper cell counts, suggests that the infection may worsen quickly and progress to AIDS if not treated right away.

The viral load test is also used to find out how well an infected person is responding to treatment. Doctors look for the viral load to go down and stay down while a person is taking anti-HIV drugs. If it doesn't go down, or if it goes up, the anti-HIV drugs may need to be changed.

Viral load testing is done using polymerase chain reaction (PCR) or other lab techniques (see above). It is not normally used to find out if a person has HIV, because of its cost and other limitations. But sometimes, a week or two after being exposed, a person may start having symptoms that suggest early HIV (see the "Primary infection" section in "The course of HIV infection"). Because an HIV antibody test will often not become positive for a few more weeks, a viral load is one kind of test that may be used soon after exposure to find out if HIV is present. It is also used to diagnose infants who are born to HIV-infected mothers.