Ordinary laboratory examinations (tests) for the identification of HIV infections determine HIV antibodies present in the blood (screening tests of the third generation) or HIV antibodies together with the virus core protein p24 (screening tests of the fourth generation). The most common test of both generations is the enzyme immunity test. Specially adapted enzyme immunity tests can also identify HIV antibodies in saliva and urine.

The first laboratory examination for suspected HIV infection is usually just a screening test. If the result of the screening test is negative, which means that the test did not identify any HIV antibodies or the virus core protein p24, the conclusion is that a person is not infected by HIV. Additional tests are usually unnecessary. If the screening test result is positive twice in a row (the expert expression for this being reactive), additional, more sophisticated supplementary or confirmatory tests must be undertaken (e.g., Western blot and immunoblot). Different countries use different combinations of screening and confirmatory tests. It is important to be aware in particular that screening tests, especially in countries in which few persons are infected, may yield false positive results. They must therefore be followed by confirmatory tests, which recognise falsely positive results of screening tests or confirm the actual presence of HIV antibodies in the blood.

The following must be known about screening tests for HIV:

• On average, the test result is positive in 18 to 25 days after infection by HIV, which means that enough antibodies or virus core protein p24 have formed to be detected by the test.
• Three months after infection, more than 99% of persons test positive for HIV antibodies.
• If the infection has occurred recently, the screening test result may still be negative.
• When saliva or urine is used for testing, the screening test result is positive later after infection than when a blood sample is used.