[Question #7136] Syphilis - False Positive RPR test

8 months ago
Hello Doctors,

I previously used your service to find out more about testing windows and gained information of properly testing for chlamydia and gonorrhea. I am a gay male, and I was NOT sexually active for over a year until March 16th, I received unprotected oral sex and March 20th, I was the bottom during "unprotected" sex. I tested 5 days after for baseline: everything came back negative, but the RPR test for Syphilis (titer 1:1).  My doctor was surprised for how low it was, he was confident it was a false positive.  I was retested with a FTA-ABS test and that was non-reactive.  The doctor explained how the RPR can be triggered by something else and that test is known for having false positives.  

6 weeks later, I went in for my follow-up, but the doctor only wanted to test me for HIV & HEP C - both negative. (this is when I reached out to your services, because he suggested I comeback in 6 months to confirm).  

At week 12, I began having stomach issues and pain all over my abs and back.  I went to an Urgent Care, where they tested me for H Pylori and I requested for an HIV and HEP C.  I tested negative for HIV and HEP C,  but positive for H. Pylori.  I was proscribed 2 rounds of antibiotics for 14 days ( Clarithromycin 500 mg and Amoxicillin 500 mg, both twice a day). This round of antibiotics failed and I was then put on Doxycycline 100 MG twice a day, and Metronidazole 500 mg 3 times a day, both for 14 days.  I just finished this round. I read that Doxycycline is used to treat Syphilis and this is what I am wondering...

1. Could the H. Pylori be the cause for the false RPR test? Have you seen this happen?
2.  If the FTA-ABS test is a false negative, would this amount of Doxycycline be good enough to successfully treat Syphilis?
3. If the Doxycycline treated the Syphilis, would this freeze the titer at 1:1 or would it become non-reactive on my next RPR test? If so, when would be a good time to re-test after antibotics?
H. Hunter Handsfield, MD
H. Hunter Handsfield, MD
8 months ago
Treetings and welcome back to the forum.

All is well -- no worries about syphilis, HIV, or other STI; for sure none of these is the cause of your symptoms. They have nothing to do with the sexual encounter last March.

You have a syphilis blood test situation that STD experts, for a hundred years, have called BFP for "biological false positive". It's quite common, estimated to occur in around 1-2% of the US population and probaby similar elsewhere -- in other words, in millions of people wherever you are. In a small proportion of patients, BFP is caused by another underlying medical condition, perhaps sometimes due to other infections, but in most cases there is nothing else important going on and nothing to worry about. There are no data to suggest Helicobacter pylori infection can cause BFP, but it might be possible. To your questions:

1) Discussed above. Since you have previously had syphilis blood tests without BFP results and now have BFP, I think it is possible it's related to the H. pylori. There's no way to know for sure, except that you might consider retesting for syphilis in 3 months. If your RPR then is negative, and your H. pylori treatment has been successful, the BFP situation may be gone. Discuss it with your doctor. In fact, I'm interested to know and will leave this thread open longer than usual so you can let me know the result.

2) There is no such thing as false negative FTA-ABS. It is 100% certain you do not have syphils. Also, even if somehow you had syphilis or were exposed to it, the first round of H. pylori treatment would have cured it. Amoxicillin alone would cure syphilis, and clarithromycin usually is active against syphilis as well.

3) Therefore, taking doxycycline won't have any effect one way or the other. It is impossible you have syphilis and do not need doxy (or anything else) on account of potential syphilis. As discussed above, your weakly positive BFP RPR result might go away with time. As I said above, 3 months might be about right for another blood test.  In the meantime, please don't give that test or syphilis another thought. (On top of all that, you had an exposure that was very unlikely to transmit syphilis in the first place.)

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

HHH, MD
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8 months ago
Thank you Dr.! 

Thanks for confirming the HIV & Hep C. You even assured me in my previous question. 

Although, the titer for Hep C has been rising. First test was .05, second at 6 weeks was .09, and at 12 weeks was .16. Is this normal? It needs to be >1.0 to be positive. 

Also, good to hear about the FTA-ABS being 100%. I was worried the RPR test was picking up syphilis before the FTA-ABS. This is my main concern, thinking that if my antibiotics for h pylori treated syphilis, before confirming with FTA-ABS, it would throw off progress in treatment if my future FTA-ABS becomes positive for life and my RPR remains the same. 

I have an upper endoscopy on August 26th, to hopefully see if my treatment for H pylori worked. Then I’ll get another syphilis test in the future. 
H. Hunter Handsfield, MD
H. Hunter Handsfield, MD
8 months ago
With all ELISA tests (standard for HIV, HCV, HSV, etc), numerical values below the cut-off typically vary; changes make no difference as long as the result remains below the cut-off for a positve. Even if your HCV result were, say, 5 times higher at 0.8, it would make no difference. In fact, the same specimen tested 10 times could give quite different numbers.

Anyway, glad to hear you're apparently feeling reassured about syphilis. Threads normallly are closed after two follow-up exchanges like this, or 28 days, whichever comes first. But as noted above, I'll keep this thread open for up to 3 months, hoping to hear the result of a future syphilis blood test about that time. (You could do it sooner, but if your BFP result goes away at all -- which it may not -- I think 3 months is a more likely time frame.)
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