Hello Dr

On December 15th 2021 I had mutual oral sex with a woman that wasn't my partner. Concerned about the possibility of STI's I took a home test kit from letsgetchecked.co.uk (urine sample and blood taken from finger sent to a lab for testing) screening for HIV (4th gen), syphilis, hep B, chlamydia, Trichomoniasis and gonorrhea at 20 days post exposure. All results came back negative/non-reactive.

The woman I had oral sex with took a HIV test (4th gen) at 28 days and this was non-reactive. I took a follow up test at 8 weeks to hit all the test windows and put this behind me. However, I was prescribed phenoxymethylpenicillin for sinusitis (250mg tabs, 8 per day for a week) and I'm worried this may have interfered with the test results. Here's the timeline

15th December 2021 - Mutual oral sex and mutual masturbation (fluids involved) - about 3 times

4th January 2022 - First set of tests

29th January 2022 - Began phenoxymethylpenicillin

4th February 2022 - Last day of medication

9th February 2022 - Repeated same tests.

Could the course of phenoxymethylpenicillin have caused a false negative for the syphilis test (or any of the others)? I'm concerned that kissing or having sex with my partner could put her at risk. The syhpilis test I took was described as an "in vitro qualitative determination of total antibodies to Treponema pallidum in human serum and plasma" and "An electrochemiluminescence assay is performed, which looks for the presence of antibodies Treponema pallidium, the bacteria that cause syphilis. This is called a treponemal antibody test."

Would I need to repeat any of these tests?

Many thanks for your time.

With regard to Syphilis, this was a low risk exposure. In the United States and the United Kingdom over 2/3 of syphilis now occurs amongst men who have sex with men. The remaining third occur in heterosexual men and women with the highest prevalence being in persons who are using illegal drugs.  And then Syphilis is a relatively uncommon STI and particularly uncommon than women, your exposure was relatively low risk.  With regard to your penicillin, timing is everything.  The penicillin that you took might have changed the course of syphilis following exposure if you had taken it earlier (within a few days of exposure)  however, In that you begin taking penicillin more than three weeks after the encounter, if you had syphilis your test would be positive. The treponemal antibody test that you report is very sensitive and would have been positive at this time how do you acquired Syphilis. You did not. There is no need for continuing concern for further testing.

I hope this information is helpful to you. If there are additional questions or any part of my responses unclear, please don’t hesitate to use Europe to two follow-ups for clarification. EWH

Thank you so much for your quick reply. That's really reassured me :)

There's so much conflicting info, even within the NHS regarding test windows so your insight on this forum has been tremendous. Keep up the amazing work you're doing here.

Hello Dr, sorry for the follow up (this is predomininatly anxiety driven).

There is quite a bit of misleading info throughout different UK GUM clinics regarding test windows. SH:24 advise that HIV tests post 7 weeks are conclusive but my local clinic advised to wait 12 weeks. I've seen other NHS clinincs say 4 weeks.

Have you ever seen a negative Syphilis or HIV test at 8 weeks turn positive at the 12 week mark?

Thank you so much. I just have one final question. 2 weeks ago I received a handjob from the same woman and fingered her. I then masturbated myself with vaginal secretion on my hand. I've just noticed a small bump on my penis. I am prone to Fordyce spots but this is a little longer (oblong in shape). Colour wise it's not too disimilar to the rest of my penis. When erect and the skin is more taught it looks lighter. Currently it's totally painless.

Is there any risk that the vaginal secretion on my hand could have transferred herpes (or anything else e.g. warts) to my penis?

THIS 3rd response completes this thread.  Try not to worry.  EWH