Past an HIV-infection the immune system produces antibodies to fight against the infection. These antibodies can be detected by an “HIV-test”. Additionally this test detects a viral component the p24 antigen. The test consists out of two parts:
- Screening test: ELISA
- Confirmation test: Immunoblot
One should keep in mind that it takes a couple of weeks after the infection for the production of antibodies. This means that during the first weeks after the infection, the HIV-test may show a negative result, although an infection is already present. This is called the “diagnostic gap”. Due to the additional testing of p24 antigen an HIV-test normally will be positive around 3-6 weeks after the infection.
Course of the HIV-test
First of all, the screening test "ELISA" will be applied. A negative result means that no antibodies against HIV and no p24 antigen were detected. An HIV-infection can be ruled out generally if the last risk contact was more than six weeks ago.
If the ELISA test shows a positive result, antibodies against HIV and / or p24 antigen were detected. To exclude a possible wrong positive result, a confirmation test is applied, called Immunoblot. This test detects different antibodies against HIV-typical proteins. If this test also shows positive results, the suspicion that an HIV-infection is present is substantiated. If the Immunoblot test shows a negative result and the risk contact was less than six weeks ago it is necessary to undergo either additional check by PCR-test directly or a monitoring after six weeks.To rule out a possible mix-up of the blood samples or a laboratory mistake, a second test with another blood sample should be carried out.
In rare cases, in which it might be necessary to exclude an infection with HIV very early, a PCR-test is the best choice for an early detection.