Diagnosing HIV


Diagnostic testing can only be done with your consent. Pre-employment testing is now illegal in South Africa. Testing by life insurance companies is still often required, but can only be done if the client gives consent.

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 by enzyme liked immunosorbent assay or ELISA testing by 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.  These days, most laboratories screen with an ELISA that tests for antibody as well as a viral protein (p24 antigen). The p24 antigen appears in the blood a week before antibody.  This shortens the window period (time from exposure to development of a positive test). 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. Voluntary HIV testing and counselling (VCT) is offered to the public free on request at public clinics in South Africa.  Testing is done on blood from a finger prick. If the test is positive, a second (different) test will be done immediately.  Both tests must be positive 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 modern tests. False negative tests can also occur, especially if the test is done too soon after last exposure.  

Currently, home HIV tests are being sold in some chemists. Most health care professionals and the Department of Health are not in favour of this practice. One reason is that the quality of the test cannot be regulated, so that there may be a greater risk of false positive or false negative results. Also, a person testing themselves or someone else, will probably not have the information or psychological support that is gained through pre- and post-test counselling.

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 test. In South Africa, all babies of HIV-infected mothers are routinely tested at six weeks with this test.  Infants who are found to be PCR positive should immediately be referred for anti-retroviral treatment.  

CD4 count:

This test is used to determine 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. This level drops gradually in the blood over time and the immune system progressively weakens. Once the level drops below 350 cells per microlitre of blood, the patient should be assessed for anti-retroviral drug therapy. 

Viral load:

The 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. In practice, the viral load test is mainly used for monitoring response to antiretroviral drug treatment. In patients on anti-retroviral therapy, the viral load should be lower than detectable limits.  If the viral load is detectable, it means that the therapy is not working well enough.  This could be because of non compliance (patient is not taking the medication reliably) or the virus has developed resistance to the drugs. 

Reviewed by Dr Diana Hardie, clinical virologist, National Health Laboratory Service and University of Cape Town, July 2010.

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