Blood tests are commonly used to detect the presence of antibodies in HIV. Testing directly for the presence of viral RNA is also used to screen blood, but this is more commonly used to monitor donated blood or to diagnose children of HIV-positive mothers.
The most common blood test for HIV is called an enzyme immunoassay (EIA or ELISA). The sample used is blood taken from a vein, according to hivtest.org.
Enzyme immunoassays rely on the presence of antibodies to the virus. According to hivtest.org, it takes an average of 25 days to develop a level of antibodies that can be detected by tests (a process called seroconversion), but it may take up to six months.
People should not be tested for HIV less than one month after possibly contracting the virus because the test is very likely to return a false negative. Because it may take up to six months to seroconvert, people who test negative before this time may need to be tested again.
Most HIV blood tests take a number of days or more to return results. However, hivtest.org reports that quick tests can return equally accurate results in about half an hour.
There is one blood test that has been approved by the U.S. Food and Drug Administration (FDA) for HIV testing in the home—the Home Access HIV-1 Test System. Other tests have not been approved and should not be used.