Some notable details would be their apartment was very dirty. They were wearing a covid type mask because they said they were recently sick with a cold. Because of the mask i was unable to see any sores if there were any. They also asked me my sti status and if i was clean which i feel is a good sign, and they told me they get tested every 3 months but i don’t know how true that is. The fellatio was maybe 30-40 seconds. The next day (Dec4) i took 200mg doxy-pep.

Yesterday (13days post possible exposure) i was tested for chlamydia, gonorrhea, and syphilis. My results came back today all negative. 

My question is how accurate is a syphilis test at only 13days after possible exposure? I believe they did both RPR and T.pallidum ab.

And how effective is doxypep for syphilis 25 hours after possible exposure?

Starting with the title of your question and its first line:  probably you are at little or no risk of syphilis. Oral sex can transmit syphilis, but uncommonly; and of course the large majority of trans women (even the most sexually active) do not have syphilis.

Now to the details. By far the most important is your use of doxy-PEP in the correct dose of 200 mg:  that alone makes it impossible you have syphilis from this event. Therefore, the accuracy of syphilis testing at 13 days (or any other time) doesn't matter; you didn't need testing at all, except for its reassurance value. (However, if it WERE important, 13 days would be much too early. It takes at least 3 weeks and often up to 6 weeks for positive syphilis test results.)

In addition, just absence of symptoms (chancre, i.e. syphilitic ulcer on your penis) by 2 weeks after the event was nearly 100% assurance you weren't infected, even without the doxycycline.

"how effective is doxypep for syphilis 25 hours after possible exposure?" 100% or close to it. Same for chlamydia prevention. But chlamydia was even more unlikely than syphilis, because it is rarely transmitted by oral sex.

My only other comment is that how dirty the apartment was is irrelevant. Perhaps it indirectly implies a resident who is less careful about other aspects of hygiene, like sexual safety, but uncleanliness of living quarters doesn't affect actual STI transmission risk. More important by far is your partner's assurance that she is tested regularly; people rarely lie about these things when asked directly.

The bottom lines:  You were at little or no risk for any STI from this event and for sure did not acquire syphilis, chlamydia or gonorrhea.

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

HHH, MD

I suppose my only follow up question would be do you suggest a more conclusive syphilis test be taken in a couple of weeks? Or do you have that much confidence that the doxypep did its job. I only ask because i am nervous to have unprotected intercourse with my partner until i know i am clean. 

Many or most asymptomatic cases are because of difficult to observe sites -- like inside the rectum or vagina. When the only exposed site is external, like the penis, probably 90% of new infections cause obvious symptoms.