Tests

Note: If somebody thinks they may have been exposed to HIV in the last 72 hours, PEP should be considered.

Traditional antibody tests and most rapid tests are unlikely to detect HIV antibodies until four to six weeks after infection.

An HIV p24 antigen test can detect HIV infection around a week before the appearance of HIV antibodies, and fourth-generation tests combine detection of antigen and antibody. A fourth-generation rapid test is now available (Determine HIV-1/2 Ag/Ab Combo).

Viral load testing is even more sensitive than p24 antigen testing; HIV RNA can be detected in the blood two or three days prior to p24 antigen.

A person who tests positive for HIV RNA, despite negative or indeterminate antibody results, is said to have primary HIV infection. However, false positive results are fairly common. A viral load result of less than 5000 copies/ml during primary infection is probably suggestive of a false positive, because most people have viral loads above 100,000 during this time.

There is disagreement among experts about the appropriate use of viral load testing during the early days of HIV infection. Some doctors recommend testing for people with symptoms of seroconversion illness who may have been exposed to HIV. Others are less enthusiastic about the uses of viral testing for asymptomatic individuals with a possible exposure to HIV because of a low yield in actual cases of HIV infection, the risk of false positive results and the test’s expense. Moreover, while viral load testing may offer a considerable advantage over ELISAs which test only for HIV antibodies, their advantage over fourth-generation tests is slight.

In the United States some clinics use a pooled test for HIV RNA in order to detect acute HIV infections. All blood samples are first tested with an ELISA antibody test, and those which are negative or indeterminate are put forward for a nuceleic acid amplification test for HIV RNA. But rather than run separate HIV RNA tests on each of the patients (which would be expensive), the tests are run on pooled samples of blood. Samples are stored until there are at least ninety specimens to pool. This pool is then amplified for evidence of HIV RNA. If the pool tests negative, then all the specimens are deemed to be negative. However, if the pool tests positive, then smaller pools of 10 are tested until each of the HIV RNA positive specimens are isolated.¹

Any initial reactive ('positive') result will need to be confirmed with at least one other testing technology. A reactive test for p24 can be confirmed with a test for HIV RNA, but antibody and Western Blot tests may not give definitive results at this stage. Nonetheless, indeterminate results with these tests may be suggestive of early infection.

Also, assays that can identify recent HIV infection have been developed. These assays – known generically as incidence assays, ‘detuned’ assays, or as the serological testing algorithm for recent HIV seroconversion (STARHS) – have been used primarily for research on a population level since they are considered to be too unreliable for individual use.