Hi . I can see your concern and commendable that you are concerned for your partner. I need to note that we are not medical professionals and, for your protection, cannot provide medical advice or diagnostics. I do want to share with you this article on transmission risks: https://h-i-v.net/transmission-risks. You will note that for receptive oral sex the estimated transmission risk is considered "Low (less than 0.04 percent)." Of course, these are estimates and individual circumstances may vary. I noticed that you mentioned taking PREP. I'm wondering if you meant PEP, since PREP stands for pre-exposure prophylaxis, while PEP is for post-exposure prophylaxis. If, as you stated, PEP is taken for 28 days, it can be quite effective in preventing HIV infection. Of course, the only way to be sure is through testing. This article provides info on the types of tests and the testing windows: https://h-i-v.net/test-types. Hope this information is helpful. Best, Richard (Team Member)