My question is do I need to retest for syphilis? The reason I am concerned is because of the azithromyicin I took soon after exposure. Did the cause a false negative on the tppa test?

I can imagine your concern when you saw your positive RPR. But it truly was falsely positive, as proved be confirmatory Treponema pallidum particle agglutination (TPPA) test. With few exceptions, antibiotics turn tests for various infections negative by treating and curing them -- not by just making the test falsely negative. Had the azithromycin had any effect on a recently acquired syphilis infection, your RPR also would have been negative. I wouldn't be at all concerned about the thigh bump. Even with treatment, a syphilis lesion that caused swelling would not have cleared up in only five days; and syphilis is very unlikely to cause a single such lesion. Further, the skin rash of secondary syphilis is never localized but body wide and would not clear with a topical steroid cream. I'll also add that had you acquired syphilis from unprotected vaginal sex, almost certainly you would have had an obvious chancre (the sore of primary syphilis) on your penis:  you wouldn't have missed it.

From the standpoint of the exposure and your concern about having syphilis, there is no need for any further testing for syphilis. However, somewhere along the line you might consider retesting in order to see if your false positive RPR has continued. Many false positives are transient. If yours is persistent, then in the event of future possible syphilis exposures, you would want to stipulate an enzyme immunoassay (EIA) for syphilis IgG/IgM antibody, which does not have the occasional false positive problem that is inherent in RPR testing. (Up to 1-2% all positive RPRs are false, so it's quite common.) If you choose to do this, I suggest waiting 3 months or so.

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

HHH, MD

A couple of follow up questions

1. Is the TPPA test conclusive in this time frame 5/9/23 -7/25/23

2. Is syphilis prevalent in heterosexual males and females? My town is south texas has four hundred thousand people and about 400 cases a year. The media is saying it's a big problem.

2. Syphilis is increasing in frequency to be sure, including the border areas of the southwest. However, most cases continue to occur in men who have sex with men; and the large majority of female sex workers and other sexually active women remain uninfected. Even if you were to learn your partner had active syphilis at the time of your exposure, it would not change my evaluation and advice. Please don't worry further about it:  there is no way you have syhphilis.