Ordinary HIV tests do not detect the virus, but rather the specific antibodies that are produced by the immune system in response to HIV infection. Antibodies are produced from about three weeks after infection and usually become detectable four to six weeks after infection. This four- to six-week period between infection and a positive test is called the window period. In some people the window period is longer; it may take up to three months for an antibody test to become positive after they have been infected, but this is unusual. People who think that they might have been exposed to infection are therefore usually asked to wait at least four weeks before having the HIV test. Also, even if the first test is negative (i.e., no antibodies detected), a follow-up test should be done three months after the suspected exposure.

The most widely used and best antibody test is called an ELISA test (ELISA is short for Enzyme-Linked Immunosorbent Assay). ELISA tests have to be done in a laboratory. If a positive result is obtained on an ELISA test, the laboratory will confirm the result by testing with at least one different type of ELISA test. As an additional check, a second blood specimen is usually taken from the person for repeat testing.

Testing can also be done with a rapid HIV test which can be carried out by any health care professional immediately on-site in a clinic. Two different rapid tests should be used to confirm a diagnosis of HIV infection. The advantage of rapid testing is that an HIV result is available within 30 minutes.

This sort of HIV testing is very accurate. Very rarely false positives occur due to antibodies that cross-react in the testing system, but these occur less frequently with the new generation tests.

HIV testing in babies:

In babies less than 18 months old, the mother's antibodies in the baby's blood can interfere with the HIV antibody test. Therefore, to test whether a baby is infected with HIV, it is necessary to detect the virus itself. This is commonly done with a PCR (polymerase chain reaction) test.

Once a person has tested positive for HIV, a thorough medical examination should be done to evaluate their present state of health. As other STDs and TB are often present in someone who is HIV positive, additional screening tests for these diseases should be done, so that they can be treated straight away.

There are tests to monitor how advanced a person's HIV disease is. A CD4 cell count indicates what reserves of T-helper lymphocytes the person has and therefore the remaining strength of their immune system. A normal CD4 count is 800 or more cells per microlitre of blood. HIV-infected people in the early stages of the disease have a count of 200 to 500 cells per microlitre and in late phases a count lower than 200. A viral load test measures the amount of virus in the blood, which shows how rapidly HIV is multiplying and therefore how quickly the disease is likely to progress. The viral load test is also very useful for showing how the virus is responding to antiretroviral drug treatment.