Expert: John Thai, M.D. - 1/19/2008

Question
I have heard of a certain type of Antigen testing... I believe it's p24 or something like that?  I've heard that the window period after which the tests are accurate (until the immune system starts up) is 16 days.  So, if the p24 antigen test were taken on day 16 itself, could I consider it as conclusive?

Answer
Hi,

the p24 antigen test can detect primary HIV-1 infection 7 days after exposure. However, this is not recommended for general use unless it is for the purposes of blood donation and/or in rape victims. The problem with this test is that it becomes undetectable in the majority of infected individuals after seroconversion of the p24 antigen to antibodies. if you are positive for p24, then you are definitely infected with HIV-1. It does not exclude HIV-2.

Currently, the best recommendation is to have the nucleic acid amplification test (NAAT) as it can detect HIV 2 weeks post exposure. It is not yet available everywhere and is still under consideration for FDA approval.

Regards,

John Thai, MD