The p24 antigen is a protein that is part of virus,during the first few weeks after someone is infected with HIV, p24 is produced and can be detected in the blood serum. This test can be taken taken at 2-3 weeks after possible exposure to HIV. However, would be very reliable at 28 days.
Negative at 21 days is very reassuring because the antigen production at this juncture is at peak. No AG detection is certainly great news. Be ready for your whoohoo!
4th generation tests are conclusive 4 weeks after exposure, u don't need a follow up test after that!!