The encounter was protected with a FSW in a brothel in Germany however my penis was flacid when she put the condom on which was obviosuly not fitted correcrly/tightly and proceeded with oral sex, once erect we proceeded with vaginal sex. I licked her breast/nipple and neck in the encounter. I have noticed a small formation on the tip of my tongue which doesn't look typical of syphilis but has me panicked. I have tested negative at 6 weeks, then 7 weeks and 4 days. I am having another test performed at 9 weeks. I was also given a week long course of nitrofurantoin between days 5 - 12 for a UTI which is believed to have been caused from a spermicide condom reaction. 

My questions are:

1) Could the antibiotics I took have weakened the infection so it was not detectable on the tests I have previously taken?

2) can I consider the tests to be conclusive at the 6 & 7.5 week mark? NHS guidance says 12 weeks (I'm UK based)

3) I take an injection medication monthly, trade name is xolair for asthma. Could this interfere with the accuracy of the test I.e. my body making antibodies.

4) Can I proceed with normal sexual interactions with my partner i.e unprotected sex and kissing.

As you can probably tell I have been anxious around this whole situation and have been avoiding kissing my partner and using condoms during sex. 

My first comment is that syphilis is rare in women -- including FSWs -- in Germany and elsewhere in Western Europe; and your wisdom in using a condom provided excellent protection. In addition, you appear to have a common misunderstanding of the effects of antibiotics in this situation. They generally do not turn tests negative while the infection persists. Your negative test results are nearly conclusive proof you do not have syphilis. It is not possible to know whether that is because you were never exposed (most likely) or because you were exposed but the cefixime aborted the infection. The NHS guidance on time to conclusive RPR testing is conservative; while it may rarely require 3 months, the large majority of newly infected persons (90-95%) have positive results within 6 weeks.

Azithromycin is completely ineffective against syphilis these days; although effective in the past, over the past 10-15 years almost all syphilis in Europe and North America now is resistant to it. Even if you had not vomited, it was pointless to take it for syphilis prevention. However, cefixime is highly active and almost certainly would have aborted syphilis if you were exposed.

Those comments pretty well cover your specific questions, but to be explicit:

1) Could the antibiotics I took have weakened the infection so it was not detectable on the tests I have previously taken? Almost certainly not.

2) Conclusive results at 6 and 7.5 weeks? Not quite conclusive but very close.

4) Safe to have sex with your partner? Almost certainly yes. The combination of the low risk of syphilis from the exposure described, the antibiotics, and negative test results so far make the chance you have syphilis under one chance in millions. If somehow I were in your situation, I would have resumed unprotected sex with my wife several weeks ago. It would make sense to go ahead with a final RPR at 12 weeks, but mostly for reassurance -- not because of any realistic chance of a positive result.

I hope these comments are helpful. Let me know if anything isn't clear.

Firstly, thank you for your detailed and thorough response. It has certainly made me feel a lot more assured around the situation. I have tried to attach a picture of something which has developed over the past couple of days, it is in an area which I often experience ingrown hairs but I would like some reassurance on whether you would fully exclude it as being a symptom of primary syphilis I'm not sure if the image is uploading correctly as I am receiving no confirmation. A few follow up questions below:

1) The cefixime which I took was in panic to cover ghonorrhea given the UTI symptoms which I had. Guidance online says that cefixime for syphilis requires 400mg daily for 10 days, would my 600mg one off doseage have aborted the syphilis 7 days after exposure?

2) would primary syphilis show post 9 weeks (which I'm currently at) or is this too late for primary stage to show? This will ease my mind from my eye catching anything 'suspicious'

3) I think this is obvious but as you get closer to the 12 week 'conclusive' mark the certainty percentage go up. I.e 90-95% of cases at 6 weeks, so a test at 9 weeks could be seen as well above 95% of cases caught close to 99+%? I'm considering a private clinic test at 9 weeks as a stop gap for peace of mind then the NHS one at 12 weeks. 

4) what exposure risk do I have from licking the breast/nipple and neck of the FSW? Are there any risks around infection from licking a rash if it were on these body parts and I did not see it, as opposed to a lesion/sore? 

You may be pleased to hear that I have been undertaking professional counselling to deal with the anxiety around this situation and to help with moving forward from it (although I'm very close to what is considered the end point). 

Many thanks for your response so far. 

1) You are confusing the dose needed for treatment of established syphilis versus prevention. The single dose almost certainly would effectively abort incubating syphilis, i.e. prevent infection from taking hold.

2) Primary syphilis almost always shows up within 2-3 weeks, maybe as late as 4-5 weeks in rare cases. 9 weeks is much too long.

3) Your reasoning is correct, but to some extent you are ignoring your own obsession. It seems clear you'll continue to worry you have syphilis until you have a negative test result after 12 weeks. Your fears will not be any less if you test negative at 9 weeks. I see no value in such a test, or in repeat clinical evaluation at a new clinic.

4) Oral contact with skin and nipples is zero risk for syphilis and all other STIs. 

I'm glad to hear you're getting counseling. I hope it helps. 

Thank you for your follow-up responses to my questions. To update on my situation I have returned another negative test at the 9 week mark from a paid for private service. This negative test has allowed me to move on with life and I view it as conclusive, although I will still get the 12 weekly in line with the guidance. 

Interestingly I have had some thoughts around the 12 week conservative window, although I am not a medical professional I work with data and statistics and have formed a hypothesis. The European Centre For Disease Control & Prevention published their data on syphilis infections from 2022 and around 75% of cases were in MSM. I have seen data from the CDC to suggest that upto 30% of syphilis cases are also co-infected with HIV. This made me think that the 12 week conservative window is potentially based on data where co-infection is present thus meaning that anti-bodies take longer to produce due to the effects of HIV. Again, this hypothesis of mine has really made me believe the negative test result at 6 & 9 weeks allowing me to move on. 

Thank you for all of your help and guidance, it is greatly appreciated. Your service is invaluable - I'm sure it has saved countless lives when people are at crisis point with nowhere to turn. 

Beyond that, I'm glad the discussion has been helpful. Best wishes and stay safe.