HIV antibody testing
When a person is infected with HIV, the body responds by producing antibodies. However, these antibodies are not protective in nature. An HIV antibody test detects the presence of such antibodies. A positive test result shows that a person has been infected. A negative HIV antibody test result after the window period means that a person has not been infected. Nowadays we can have HIV antibody test with blood, oral fluid (oral mucosal transudate; it is not the same as spitting out saliva) or urine.
HIV p24 antigen testing
p24 is one of the proteins underlying HIV infections. p24 antigen test detects the level of p24 in the blood and is useful in diagnosing early HIV infection. However, as the level of p24 drops rapidly after the HIV antibodies have been produced (about 4 weeks after the HIV infection), p24 antigen test is not always reliable for diagnosing HIV infection after its early stage.
Rapid test takes only 20 minutes for the result, but it is only a screening test (a preliminary test). Any positive rapid test result should be followed by confirmatory test of venous blood conducted by a certified laboratory. Conventional HIV blood test (drawing blood from vein with a syringe) takes around 1 week for the result.