To go straight to it I had a potential exposure to syphilis/hiv 89 days prior to me writing this question. 

I met somebody off of grindr to perform oral sex on him. He also performed oral sex on me (male) but only for a minute or 2. The oral sex I performed on him probably lasted around 10-15 minutes and ended in him ejaculating in my mouth and me swallowing. I didn't see any chancres or sores on his penis but I didn't do a "thorough" check. About a week post exposure I had a couple of ulcers in my mouth as well as a tonsil enlarged on 1 side (this could of been there prior to the blowjob to be honest). I completed 2 lots of 5 day courses of penicillin V. On day 17 I had what I thought at the time could be a oral chancre tucked under my tongue in the fold between it and the root under my tooth (hard to explain this location exactly) I was really stressed about it and kept on touching it until I caused damage to the area and it was covered in dead skin (most of me believes this wasn't a chancre). Day 38 post potential exposure I had a full sti screening and everything came back negative. The test was oral swab, urine and blood draw from vein

My main questions are

1. I am 90% confident EIA as this is standard practice in the Uk, is this conclusive at 38 days post exposure.

2. Would my antibiotic treatment of interfered with the test result and potentially caused a false negative? (Finished antibiotics around 7-14 days prior to to test)

3. Was my HIV test conclusive at 38 days (I imagine this was 4th or even 5th gen)

I had a potential hiv exposure last year that caused me a lot of anxiety (I was okay in the end) so I'm kicking myself I've let this happen again.

Thank you :)

The encounter you describe was low risk.  Most people do not have STIs, including HIV and most solitary encounters with infected partners do not result in transmission of infection.  This is particularly true for oral sex with is lower risk than genital-genital or ano-genital contact.  I do not have an explanation for the oral lesions you have noted but I am confident they were not syphilis- you proved that with your blood tests at day 38.  If the lesions had been syphilis, the blood test would have been positive. In response to your specific questions:

1. If it was an rpr test I got given (I'm not sure how likely that is) are the results still conclusive?

2. Somebody else told me that penicillin V can bs effective against syphilis however I was skeptical as what I found online stated otherwise. Are you able to expand on this at all?

2.  Penicillin is the preferred medication for syphilis treatment. It comes in many forms. The recommended form used for treatment is an injection of Ben Ethan penicillin G however penicillin V is taken for a week or more. It would certainly prevent development of Syphilis..  EWH

1. Since my 4th gen was so close to 6 weeks (only 4 days out) could i consider the result 99.9% reliable or more?

2. I've seen online the giving a blowjob poses a 1/2500 risk of contracting hiv if given to a positive partner. Is this an accurate stat or is the risk lower than this?

1.  Yes, I would consider these results conclusive and would not feel the need for further testing.

2.  Most experts calculate the average risk of performing oral sex on an HIV infected, untreated partner is being less than one infection per 10,000 encounters.

I hope the information I’ve provided has been helpful to you. This thread is now complete and will be closed shortly. EWH.