9 weeks ago, I engaged in mutual masterbation and protected oral sex with a man who was also in relationship with a women but who had engaged in MSM behaviours in the past. 

He did not perform oral sex on me and I did not come in contact with any of his semen as he 'finished himself off'. At some point, I may have also stimulated his anus with my finger. To my knowledge, I had no significant cuts or other injuries on my hands or penis at the time. However, I work outside alot and therefore my hands are prone to cuts and scrapes which tend to heal reasonably fast. 

I had an STI test 4 weeks after this encounter. Due to Covid, it is very difficult to get a clinic appointment so I had to use the SH:24 service where they send a test kit in the post and you collect a finger-prick blood sample to test for HIV and Syphilis and a urine test for Chlymdia and Gonorrhoea. All of these results were negative. 

I have previously been reassured to read that 28 day 4th Generation HIV results were highly accurate and in many cases definitive. 

However, recently BASHH guidelines were updated and increased the testing window period to 45 days. This has caused me some anxiety about whether or not I could rely on my test result. Firstly, is the finger-prick blood in the post method as accurate as blood drawn from the arm and secondly does my test at 28 days rule out HIV? 

A few days ago, I got out the shower and noticed that at the base of my glans head, there was a lot of redness.  It was very red and smooth and covered half of my glans head. This has been improving over the last couple of days but when in the shower last night, the skin peeled off and the redness underneath appeared again. 

Apart from this encounter, the only other thing I can think which may have caused it would be that I used an emollient cream to wash a couple days before which I have since discovered was 1 month out of date. 

Is this a symptom of Syphilis? How accurate would my finger prick test for at 28 days be? 

If you were going to develop a lesion of syphilis, the redness noted on your glans would progress to form a shallow, painless ulcer.  The redness would not simply resolve as seems to be the case for you.  Further, it would be most unusual for a syphilis lesion to appear 8-9 weeks following exposure which seem to be your situation if I follow correctly.

Regarding the blood spot test in comparison to syphilis and HIV tests done on blood contained in a tube of drawn blood, the results are nearly equivalent although the blood spot test may be slightly less sensitive, particularly early in the course of infection.  HIV test results are not entirely conclusive until 6 weeks following an exposure.  I would anticipate that your negative tests at 28 days make both syphilis and HIV most unlikely, particularly given the virtually no risk nature of your exposure.   Personally, I would probably not both to test again but if you wish to be entirely, 100% sure, it may be worthwhile to repeat the blood spot tests although I would be surprised if the result were not negative once again.  

Sorry to waffle a bit on the response but the performance data are what they are.  Hope this helps.  EWH

Thank you for your swift reply. 

I might be misunderstanding, but it was not a blood spot test for HIV and Syphilis. I took a finger prick with a small lance and then dripped 16 drops of blood into a small tube which was then sent off in the post to a laboratory. Apparently it is a 4th Gen HIV and an Igm or something test for Syphilis.

Thank you again for your reply.