[Question #3240] Syphilis Testing

91 months ago

Hello,

About 15 years ago I had an unprotected sexual experience. A woman and I were grinding our genitals and then I briefly inserted my penis into her vagina. I waited a few months and then went and got tested for HIV, chlamydia, gonorrhea, and herpes (the blood test, not the swab). I was tested for everything but syphilis because for some reason the place I went to did not offer a syphilis test. A few weeks after the encounter I did get what looked to me like a pimple on my penis. I popped the pimple and it went away.

After putting it off for almost 15 years, I recently decided to talk to my doctor and get tested for syphilis. The doctor ordered two tests: the RPR test and the FTA-ABS. It's my understanding that the FTA-ABS will detect past or present syphilis infections. However, I was doing some research and stumbled onto an article on the National Center for Biotechnology Information's website titled "VDRL Test and Its Interpretation." In the article, the writer states that "2-4% of patients with late syphilis will show negative results with treponemal tests also." The writer goes on to suggest a polymerase chain reaction (PCR) test to establish the syphilis diagnosis. I Googled the test and couldn't find very much information. If I were to have syphilis, after 15 years I assume there is a good chance it is in one of the later stages.

So my question is if both the RPR test and the FTA-ABS come back negative, can I conclude that I do not have syphilis? As a lay person, am I misunderstanding the article? If so, please explain how. Or, is there still a chance I could be in that 2-4%?

I look forward to reading your response. Thank you for offering this service.

H. Hunter Handsfield, MD

91 months ago

Welcome to the forum. Thanks for your question, and thanks for your confidence in our services.

First, you describe an exceedingly low risk scenario in regard to syphilis, especially if you are in the US or other industrialized countries. Roughly 15 years ago (early 2000s), syphilis was very rare in heterosexual women. At the time (and still now) the large majority of cases were in men having sex with men. In addition, "grinding" plus the very brief unprotected vaginal sex carries little risk. My guess is that the clinic where you had your STD evaluation didn't test for syphilis because they understood the risk was low. (At that time, we offered but did not insist on syphilis testing among patients attending my STD clinic.)

But most important, turning to your specific question: you have misunderstood the implications of something you read. It is not true that the RPR misses active syphlis. If the RPR is negative, and if there are no symptoms to suggest active syphilis, no experts recommend also doing an FTA-ABS. Previoius syphilis that has resolved, or that was effectively treated, can give a negative RPR with postiive FTA-ABS, which once positive remains positive for life. But a negative RPR alone is sufficient evidence against active syphilis. PCR for T. pallidum is not relevant. It's not a blood test at all, but an assay that sometimes is done on suspected lesions or spinal fluid.

Having said all that, these are all the wrong test anyway. Most labs' routine syphilis screening test is no longer RPR, but an IgG ELISA. This is a "treponemal" test, i.e. equivalent to the FTA-ABS. It also is positive for life and is the cheapest and quickest type of syphilis blood test. A negative result, which is what you definitely can expect, will rule out both past and current syphilis with 100% certainty. However, if for some reason your doctor indeed ordered an RPR, the lab may do it, but probably will not do the FTA-ABS if the RPR is negative -- as expected.

I hope this information is helpful. Let me know if anything isnt' clear.

HHH, MD

---

91 months ago

Thanks for the detailed response. I looked at the lab orders and there are two tests listed. A test called Treponemal Pallidum AB, IFA, serum and the RPR test. I'll have to look at the test results to see what specific testing methods are used and if both tests are actually done. If the doctor specifically ordered the above tests, is there a chance the lab will disregard the RPR test and run the IgG Elisa because it's more current?

When you say "active syphilis," does that mean that you can also be infected but the syphilis has somehow become inactive, and if so, how would that work? Also, when you say previous syphilis that has "resolved," do you mean that the body has somehow cleared the infection by itself, like how Keith Richards is rumored to have cleared hepatitis C? So you've had the disease but your body managed to fight it off and now you don't have it.

It sounds like I have misunderstood a number of things I have read. I also read that in the later stages of syphilis the RPR test will sometimes report false negatives. If this is correct, wouldn't that mean you could have syphilis at some later stage, take the RPR test and get a negative result, but still have syphilis? Or, am I missing something here? I'm admittedly pretty confused now. If I only end up getting the RPR test and it is negative, would that result rule out syphilis at any stage? Or, would I need to take the IgG Elisa test you referenced to do that?

Lastly, if the lab does end up doing the Treponemal Pallidum, AB, IFA, serum test the doctor ordered and it is negative, am I correct in assuming a negative result means I don't have syphilis and never did?

I appreciate your first response and apologize for how ill-informed I was/am.

H. Hunter Handsfield, MD

91 months ago

Although terminologies differ, I'm pretty sure T. pallidum Ab IFA is the ELISA test I mentioned above. Probably they do the initial test only, then automatically do an RPR if the first is positive.

Most human infections are cured by the immune system, and that is true of syphilis probably ~90% of the time. However, the blood tests remain positive. The term "inactive" is meant to encompass both resolved infection and persistent (latent) infection without symptoms. A negative RPR is definitive evidence that someone has either no syphilis at all or, if s/he did, that the infection is not active i.e. not causing disease or harm in any way.

You're confused only because you seem to be overthinking all this. "If the lab does end up doing the Treponemal Pallidum, AB, IFA, serum test the doctor ordered and it is negative, am I correct in assuming a negative result means I don't have syphilis and never did?" Exactly right.

And I'll also remind you of my opening comment about how rare syphils is and was at the time of the sexual encounter you asked about. Absence of symptoms to suggest syphilis in the weeks after the exposure (usually in the form of a penile sore that persists without healing for several weeks, and later a body wide skin rash) also is good evidence you weren't infected. There is no realistic chance of syphilis from the exposure you asked about above, well under 1 chance in a million. This really isn't worth the amount of mental energy you have been devoting to it!

---

91 months ago

Thank you for the clarifications and for the new information you provided.

I thought the results would specify what type of test the Treponema Pallidum AB, IFA actually was, an IgG ELISA or an FTA-ABS. I assumed the test was an FTA-ABS because when I Googled "Treponema Pallidum AB, IFA," the FTA-ABS test was what came up.

In either case, regardless of the specific type (FTA-ABS, ELISA, or something else), if someone takes a treponemal syphilis test and the result is negative, unless the person has been recently infected, he or she does not have syphilis and never did, correct? In other words, once someone currently has or has had syphilis, the treponemal tests are positive for life, right?

H. Hunter Handsfield, MD

91 months ago

"if someone takes a treponemal syphilis test and the result is negative, unless the person has been recently infected, he or she does not have syphilis and never did, correct? In other words, once someone currently has or has had syphilis, the treponemal tests are positive for life, right?"

All correct. You said it very well.

That concludes the two follow-up questions and replies included with each orginal question, and so ends this thread. Do your best to stop overthinking all this, searching the internet, and worrying.There is no way you have syphlis. This really is a waste of your emotional energy and time. But I do hope the discussion has been helpful.

---