[Question #11064] Syphilis testing

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17 months ago
A while back I had sex with an independent escort.  Fellatio w/ condom and vaginal sex with the same condom. I believe the condom was intact the whole time, but I admit I'm inexperienced.  It stayed on the whole time.
I was on doxycycline from a week before the encounter to 3 weeks after the encounter.  100mg 2x /day for acne.
9-10 weeks later I did comprehensive STD testing, including syphilis (RPR, RFX, Qn RPR/Confirm TP), and all came back negative.  I did follow up testing 5.5 months later and same results.  The lab work was done by LabCorp.
About 1-2 weeks after the escort I had sex with my LTR without protection.  Is there any way at all I could have passed syphilis to her without knowing it, and the doxycycline masked symptoms in me, and then everything cleared up before I got the test? Neither of us noticed a chancre or anything, but I understand that sometimes STDs don't have symptoms or the symptoms aren't noticed for some reason.  I think I heard that doxycycline kills syphilis, and that if you get a negative RPR that means you have never had syphilis, but I'm not sure I'm understanding that right so I'm looking to the experts for advice.
Would it make a difference if I was wrong about the doxy dose and it was only 50mg 2x /day?  Would your judgment be the same for other bacterial STIs?
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H. Hunter Handsfield, MD
17 months ago
Welcome to the forum. Thank you for your confidence in our services.

You've almost answered your own questions. There is no possibility of acquiring syphilis while taking doxycycline -- certainly not at 100 mg twice daily (a higher than usual dose for acne) and probably also effective at 50 mg twice daily. Doxycyline 100 mg twice daily is actually now recommended as the drug of choice to treat syphilis in the common circumstance when long acting penicillin isn't readily available. (There's currently a world wide shortage of this kind of penicillin.) Doxycycline does not merely mask symptoms, it is 100% reliable in preventing and effectively treating syphilis. Having taken it, there was no point in having the syphilis blood tests you report. (When doxycycline isn't involved, a negative RPR is proof that active syphilis is absent, but does not rule out the possibility of past syphilis.) 

Beyond this, you had a sexual exposure that was no risk for syphilis anyway. It's very rare in escorts (i.e. expensive female sex workers by appointment, as opposed to brothel workers, bar pick-ups, etc); and condom protected sex is close to 100% protective even if your partner was infected.

For those reasons, there is no possibilty you have syphilis and equally no possibiity you infected your regualr partner. Don't worry about it.

I hope these comments are helpful. Let me know if anything isn't clear. You apparently asked your previous question several months ago be deleted. Let's plan on this one remaining available, please. The moderators often glean useful information about users' current questions by reviewing their previous ones. Thank you!

HHH, MD
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17 months ago
Thank you Dr. Handsfield.  That's very reassuring.  I have just a couple of questions about your answer:
Would your judgment be the same about other bacterial STIs like chlamydia, gonorrhea, NGUs, etc.?  Is doxy protective against them as well?
You said that the RPR test proves that "active" syphilis is not present. Does that mean a person could have latent syphilis but the RPR would not detect it?  The notion that a STD can be present without symptoms is concerning.  Suppose a person is being seen for some condition and the doctor orders an RPR test to rule out syphilis as a cause - could the RPR come back non-reactive but the person would still have latent syphilis causing the condition?  Would the doctor have needed to do different testing to REALLY rule out syphilis?
Thanks again.

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H. Hunter Handsfield, MD
17 months ago
Doxycycline is 100% effective against chlamydia and most NGU, and reduces the frequency of gonorrhea after exposure by about 50%. But both the very low risk and absence of symptoms are proof you don't have NGU gonorrhea, regardless of taking doxycycline.

Your RPR result means you do not have syphilis. Conceivably you had past syphilis that was treated and cured; sorry if the word "active" confused you. The test is 100% proof you cannot possibly have any syphilis infection at this time; even if you had it years ago, the doxycycline taken recently would have cured that too. Your current RPR "REALLY" rules out syphilis, with no further testing needed. And syphilis can never cause symptoms in presence of negative test results. Do your best to stop splitting hairs and over interpreting things:  you don't have it, period; there is nothing more you can possibly think of that could change that fact!
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