Is STD Testing Conclusive At 19 Days Post Exposure?

I recently administered oral sex on a male. He did not ejaculate , but I m pretty sure there was precum released during the oral. His reported HIV status is negative. I followed up with STI testing 19 days after exposure -- and all test results returned negative. I plan to follow up again at the 3 month mark this week. how likely is it that the 19 day test is a good indicator of a negative HIV status?