[Question #1439] Possible false negative on chlamydia test?

45 months ago
Greetings doctor,

2 weeks after I made the mistake of having unprotected sex, I went to a planned parenthood clinic to do an STI check. My tests came back NEGATIVE. however, the week before I went in for my check up, so one week after being intimate, I had to take erythromycin 500 mg once or twice daily (can't remember) for a bad tooth infection/abscess above my bridge. 

I am wondering could the use of antibiotics have caused a false negative? 
I called planned parenthood today and all they could say was that if someone is infected and treated they usually wait 3 weeks to retest because of the posibility of a false positive but not negative. Also, the planned parenthood I went to uses NAATs to test for chlamydia and I know that method is very sensitive and specific but I am still worried that the antibiotics may have caused a false positive. 

A month later I went back to my country and ended up once again having unprotected sex with an old classmate. Within 48 hours I went to the gyno over there and they did some immunofloresence testing for chlamydia and it came back positive. I am torn and cannot figure out if I actually had the infection before I went and did not get treated because of a false negative, or did I actually receive it from the second guy back home and the bacteria showed up within two days? 

Thanks for your time. 
H. Hunter Handsfield, MD
H. Hunter Handsfield, MD
45 months ago
Welcome to the forum. I believe you're female, although you never say so explicitly.

Erythromycin could treat chlamydia and make a test negative. Usually it's not a false negative, however -- but a true negative because the drug cured the infection. But there is a chance it could have temporarily suppressed it, causing a false negative result.

On the other hand, false positive results are rare. But it depends a bit on the test, and "immunofluorescence" is an older, less reliable test. The main tests done today are DNA/RNA tests, also called nucleic acid amplification tests (NAAT). False positive results with NAAT are almost non-existant.

Having said all that, my advice is that you need to speak with both partners. It is certainly possible you have a chlamdial infection, and that you had it at the time of your earlier test. In other words, from the information you have provided, it is possible either (or even both) partners were/are infected and both should be treated. That's what we would recommend had you been a patient in my clinic. As you imply, it would be unusual to have a positive test only 2 days after exposure, which would exonerate partner no. 2. But it isn't possible and you shouldn't risk his health by saying nothing.

I know this advice might be disconcerting, and not what you hoped to hear. But that's how I see it. Please let me know if anything isn't clear. Also, if you can get more detail on exactly what tests were done for chlamydia, on both occasions, that might help sort things out a bit more.

Finally, another bottom line, which perhaps you can predict, is that there is a reason condoms are recommended routinely for sex with new partners! Perhaps all this will be a learning opportunity. (I hope this doesn't come across as an unwanted lecture. I don't mean it critically. It's just common sense.)

Best wishes--  HHH, MD

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45 months ago
Thank you so much for your response. 

You are right, I am a female who also makes very stupid mistakes :( 

To be clear, I had no intercourse in between the last time last year when I was tested negative to the first unprotected incident that happened this September.  So two weeks after that encounter I got my chlamydia and gonorreah test, they used NAATs for that which came back NEGATIVE. I think 3-4 days prior to getting my test is when I finished taking erythromycin.  Do you think chances are higher that it was a true negative, or a higher chance  that the antibiotic just suppressed the bacteria and caused a false negative? 

After my second incident which happened overseas and got tested for it there using immnofloresce testing is when my test for chlamydia came back positive. I was given doxycycline 100 mg twice a day. When I returned to the states I was still taking the antibiotics. 3 days after finishing my treatment I went to check again and my test here came back negative. 

I did reach out to both guys to let them know what's going on. The first guy (here in the states) is convinced that he is clean and was reassuring me that it wasn't from him (although I don't think he actually got tested) 
I just feel so confused and anxious over this situation. I know I made a mistake, but I'm trying to figure out when I really did contract it. I'm also not sure how realiable the tests in my country are and if they could really detect the chlamydia after 2 days or did they just tell me it was a positive so I could go back after treatment for a check up and make more money (many people back home complain that doctors do this)

I understand that my questions seem to be repeated but I just wanted to clarify once again.

First test in America, used NAAT (stopped taking erythromycin) 3-4 before test, NEGATIVE result.

second test overseas, after 2 days of unprotected sex came back positive and treated with doxycycline. 
H. Hunter Handsfield, MD
H. Hunter Handsfield, MD
45 months ago
Thanks for the clarifications.

Your first tests really can't be interpreted. Probably you didn't have either gonorrhea or chlamydia when tested, but there is no way to know whether you might have been infected but were cured by erythromycin. However, it sounds like your first partner is on top of things and probably not infected, so most likely you were not either.

My understanding is that immunofluorescence testing is done more often outside the US. But as noted above, it is prone to give false positive results, and I'm guessing that's the case here. Two days probably is too soon for a positive result. 

In summary, it seems most likely you never had chlamydia, but it's not possible to be certain. The safe thing is to be sure the second partner is treated (and, ideally, tested). But I'm afraid you're never going to know for sure whether you were infected or, if so, from whom. However, I would say the last doesn't matter very much at this point.

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45 months ago
Thank you so much for your clarifications Doctor. 

I understand that it's a complicated situation and hard to know whether I really had it and from whom. 

However, in your opinion do you think that the first test using NAATs which came back NEGATIVE was most likely a true negative or false negative due to erythromycin use prior to testing?  I know you said that if there was something there then the erythromycin cleared it, but how likely is it that the erythromycin suppressed the bacteria and gave me a false negative?  
H. Hunter Handsfield, MD
H. Hunter Handsfield, MD
45 months ago
These are just restatements of the same questions already answered. Sorry, but it's impossible for me to judge whether you were infected with a temporarily suppressed test, or not infected during the first encounter. I slightly suspect the last, i.e. never infected; and that your second test was a false positive. But as I have said before, it simply is not possible to be certain.

But at this point, what does it matter? While I understand your natural curiosity, at this point it makes no difference in your health or that of your partners. The only remaining issue is whether your first partner should be tested despite his confidence he isn't infected. But you've done all you can by informing him of the situation. The rest is up to him, and you are not responsible for his health or that of other partners he might have.

So my advice, once again, is that you accept the remaining uncertainties, move on, and not give any of this business another thought. There simply will never be clearer answers than you now have.

That completes the two follow-up comments and replies with each initial question and so ends this thread. Take care and stay safe -- and please use all this as a teaching moment about condoms.

Best wishes.

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