You can prove the negativity at 3 months after you got pricked. When risks exist, that will be the period at which a negative result is conclusive by medical guidelines. When we say no need to test, that's because the result will come out conclusively negative anyway.
would hiv show up on a 2 or 3 week p24 test?
No, you couldn't because it wasn't infused and you thus need no testing.
sorry I meant p24 test...