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02 May 2003

HIV antibody tests

There are three possible HIV antibody tests: HIV ELISA tests, HIV rapid tests and saliva and urine tests.

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There are three possible HIV antibody tests: HIV ELISA tests, HIV rapid tests and saliva and urine tests.

1. HIV ELISA tests
Tests most commonly used to diagnose HIV infection are those that detect HIV antibodies. Two of the best-known HIV antibody tests are the ELISA (enzyme-linked immunosorbent assay) and the Western Blot tests.

Antibodies are small proteins made by your immune system whenever a new infection invades the body.

For example, if you are infected with herpes, antibodies against herpes are made to help fight off the herpes virus. Similarly, HIV antibodies are produced when a person is infected with HIV.

HIV antibodies are present in the blood and in lower concentration in other body fluids such as saliva.

The best tests to detect HIV antibodies are the ELISA tests, which are extremely accurate in reacting to the HIV antibodies that are formed by the immune system in an unsuccessful attempt to protect the body against the virus.

HIV antibodies can usually be detected in the blood four to six weeks after infection. The antibody tests will therefore only become positive approximately six weeks after infection. (In some cases it may take much longer.)

The ELISA HIV antibody test is the most popular and commonly used test. It is widely available and reasonably cheap.

The ELISA test (as well as the Western Blot test) are laboratory tests, which means that blood must be drawn and sent to a laboratory where they use specialised equipment to test the blood. The results may therefore take anything from a day to a few days depending on where the test is done (in private practice in the city, or in a rural clinic where it has to be sent to a laboratory in the city).

The ELISA HIV antibody test is very sensitive and reliable, and produces very few false negative results. Because false positive results (where a test result is positive, while the person is actually HIV negative) can occasionally occur, an HIV positive test result should always be confirmed by means of a second test.

To make the diagnosis even more accurate, a sample that is found to test positive with one ELISA test will usually be re-tested with a second, different ELISA test.

Only if both tests are positive will you be regarded as being HIV antibody positive. Even then, it is recommended that a new blood sample be sent to the laboratory for repeat testing.

If this procedure is followed, then the accuracy of the diagnosis of HIV infection is very close to 100%.

A positive ELISA test is usually confirmed by means of another subsequent ELISA test (or with any of the other types of tests which will be discussed later on).

2. HIV rapid tests
The disadvantage of sending a blood sample to a laboratory for HIV testing is that you will usually have to wait for a few days and must return to the clinic or testing centre to get the results. For this reason, rapid tests to detect HIV antibodies have been developed.

These tests are simple to perform and do not require any laboratory equipment so that they can be done by a health care professional at the clinic or testing centre or at the bedside of a patient.

A rapid HIV antibody test is a blood test, which can be done by pricking the finger with a lancet, and applying the test tube. The result is ready within 15 minutes.

Rapid tests are very useful for the diagnosis of HIV infection in rural or isolated areas that are far removed from diagnostic laboratories and where clients often cannot afford to come back for test results.

These tests are relatively cheap and they demonstrate a high rate of reliability if they are correctly used. The accuracy of rapid tests is variable but some tests are just as good as an ELISA test.

As with ELISA testing, the accuracy of a diagnosis of HIV infection using rapid tests is higher if two rapid tests both give a positive result. All positive rapid HIV results should, however, always be confirmed with a second rapid test (from another batch) or with a laboratory-based ELISA antibody test. A second confirmatory test will, however, not be necessary if a person presents with clear symptoms of immune depression.

Rapid HIV diagnostic home-kit tests are also available, but these tests should be used with extreme care. It is not advisable to do the home test without proper pre-test and post-test counselling. The results can also be incorrect if the testing instructions are not followed exactly, if the test has not been stored at the required temperature, if the expiry date of the test has been exceeded, or if tests of a low quality are used. A positive result obtained from a home test should always be confirmed by a subsequent laboratory test.

3. Saliva and urine tests for HIV
There are ELISA tests and rapid tests that can test for HIV antibodies in saliva or urine instead of in blood.

The advantage of these tests is that a blood sample is not needed, so it is not essential for a qualified health professional to collect the sample, and there is no pain involved.

Testing for HIV antibodies in saliva or urine is slightly less accurate than testing a blood sample.

 
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