The risk of infection depends on a number of things- including the viral load ( quantity of virus) that the guy had, what specific activities you engaged in ( oral, anal sex etc), use of condoms, exchange of fluids/ blood, etc. Even still, the risk from having sex on one occasion, is still fairly uncommon- like 1 in 1000 for penile/vaginal intercourse.
We rarely do check the p24 test in the United States as it is not as accurate as other more standard tests ( ELISA and PCR). While it may be positive early after infection ( within a few weeks) I would not rely on its results one way or another.
I would recommend ELISA and Western Blot testing at 1 and 3 months after your exposure.
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