Low syphilis titer: new infection or something else?

[curiouskitten2020]

Hello, I am a 34 year old gay male. In late 2018, I identified an ulcer on my penis and sought immediate treatment for potential syphilis. My PA ordered a bicilin injection and blood work which confirmed an early syphilis infection (RPR titer 1:2). Future syphilis blood work (3 months after treatment) detected nothing in RPR/syphilis blood work. In three follow-up regular STI screening (no symptoms) panels, no antibodies detected, all clear. Most recent being in late February 2020. Now, I am in a monogamous relationship which began at end of January 2020. My partner and I both have been diagnosed with syphilis in the past (his diagnosis was 4 years ago). Both of our STI panels were clear in late February 2020 and neither of us were unfaithful (I believe him, COVID means you spend a LOT of time with your partner).

Two weeks ago, I went to my doctor for follow-up blood work unrelated and he ran a usual batter of tests, including syphilis, HIV, etc. This time, my syphilis RPR came back positive with a 1:4 RPR ratio. However, I understand this can sometimes be a false positive especially for people that may have autoimmune disorders or a history of mononucleosis (I have both). I sent my partner to his doctor for blood work and now he comes back with a 1:8 RPR titer. Neither of us at any time in the last six months have presented with any chancres/ulcers, rashes, unexplained fevers, headaches, etc. Both of us will be treated accordingly. As I understand, these are low titer levels and may not possibly indicate a new infection.

My question is: is it possible that an undetected case of syphilis slipped through BOTH of our blood work analysis in February and that we were BOTH asymptomatic to bring in a new infection? Is this possible to be a false positive and the RPR test is picking up other antibodies for both of us? We've both been monogamous since late January 2020. he had other partners in January and I had one in early January. However, I would suspect that any indication of infection would be picked up 4-6 weeks later in our late February/early March blood work. I appreciate your assistance and insight into this. Thank you.

[auntiejessi]

"is it possible that an undetected case of syphilis slipped through BOTH of our blood work analysis in February and that we were BOTH asymptomatic to bring in a new infection?"

Yes, definitely, depending on when your most recent exposures were before becoming monogamous at the end of Jan. It can take up to 90 days to show positive on blood tests, though most will by 6 weeks. Even within the 6 weeks, it's possible that your test at the end of Feb (especially given that it was a short month) missed it.

As to the symptoms, since it sounds like you were both treated in March, it sounds like you both would have been treated before you'd get the secondary stage rash, but it is strange that neither of you noticed a chancre. You've had one before, so you know what it looks like. Is it possible that your partner had one that he missed, and you perhaps had one orally that you missed, if you perform oral on him? Or vice versa? (You don't have to give me these details, I'm just offering possibilities for you to think about.)

So I can see you getting a false positive, given your autoimmune disorders, but it's odd that you wouldn't be getting the false positive always. Does your partner have any reasons for a false positives? Given that you both went from negative tests to positive, I'd have to say it's a new infection for both of you, though, but obviously, I'm guessing as much as you are.

Did I answer all your questions? We're here if you have more.