There are three main types of tests for testing the blood for HIV infection. These include tests to (a) detect antibodies; (b) identify HIV itself and estimate the number of HIV virus in the body; and

(c) provide an estimate of the number of T cells in the blood. Of these three types, the most common are the tests to detect antibodies. Detailed below are the tests in each of these categories:

Tests to detect antibodies: The most common test to detect HIV antibodies is the ELISA test, which is a short form for Enzyme Linked Immune Assay. ELISA test is preferred as an initial test for HIV testing mainly because it is simple and sensitive. It is therefore suitable for testing large number of blood samples. It is important to remember that there are several ELISA kits available commercially but not all of them have been manufactured using recommended guidelines. It is therefore desirable that the ELISA test be performed in laboratories approved of supported by the National AIDS? Control Organisation. These centres use only standard kits.

The Government of India has recommended that a person should be suspected to have HIV infection only if two consecutive and separate ELISA tests have indicated the presence of antibodies. In case the ELISA test indicates presence of antibodies, further tests to confirm HIV infection are recommended. These tests axe expensive and therefore many medical practitioners recommend three consecutive ELISA tests to be done instead of doing a confirmatory test. If each of these tests indicates presence of HIV antibodies, the person is said to have HIV infection.

A person infected with HIV develops antibodies to it only after about three to six weeks. The period between the actual infection and the time when antibodies appear in the blood is called the window period. ELISA test will be negative if the blood is tested in the window period.

Other tests to detect antibodies include rapid tests such as “dot blot ” or “immuno blot”, dipsticks, etc., and simple tests. These tests are not routinely recommended because of the higher probability of laboratory errors and higher costs as compared to the ELISA tests. Rapid tests give results within fifteen to thirty minutes only. Even if the test result is positive, it is necessary to do ELISA and Western Blot test for confirmation. This is because rapid tests often show false positive results.

Urine and saliva antibody tests have also been developed to detect HIV antibodies in the urine and saliva respectively. Both these tests however do not replace the ELISA test.

The most common test used to confirm HIV infection is the Western Blot test. This test also detects antibodies to HIV. A Western blot test is said to be positive if the test shows reactions to the antigens of at least two of the following components of the virus: P-24, GP-41 and GP-160. A negative test is one which does not indicate antibodies to any of the above components of the virus. In case there is reaction to one or more antigens only, or if there is weak reaction, the test results is said to be doubtful.

It is important to remember that about fifteen per cent people who are not infected with HIV can also have doubtful test result. This is why Western Blot test is recommended only after two consecutive ELISA tests have been positive.

In India, standard ELISA and Western Blot test facilities are available in several major hospitals.

Tests to detect HIV: The most common test to detect HIV itself is called polymerase chain reaction or PCR test. These tests can indicate the presence of the virus in new born babies or adults within a week of their getting HIV infection. The PCR tests can also estimate the number of viruses in the blood and are therefore used to assess the progress of the disease.

Tests to estimate the number of T cells in the blood: Estimating the number of T cells in the blood or the total CD4 count in the blood is used for identifying the stage of HIV infection, plan the most suited treatment option and establish the diagnosis of AIDS. It can also be used to decide whether specific preventive measures are desirable for opportunistic infections or not. This is because a decline in CD4 cells indicates increased risk of getting infections.

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