[Question #14007] Why discrepancies in testing window guidance?
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1 months ago
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Perhaps a bit academic, but I’m wondering why there are such meaningful discrepancies in guidance on how long to wait for an STI test. Less so for syphilis, HSV, and HIV. But much more so for gonorrhea, chlamydia, and trichomoniasis.
In your forum, you often say NAAT urine testing results are reliable 4-5 days post the last exposure. But a lot of online guidance says tests are not reliable until 7 or even 14 days post exposure. Why is there such a discrepancy? And what research or data gives you confidence on the shorter time windows? This isn’t to challenge your expertise (as I believe you), I’m just trying to understand where the guidance comes from.
I can see an argument (mentioned elsewhere) for orgs to try and be conservative to ensure a test’s sensitivity goes up to ensure fewer false negatives. But I can just as easily see an argument for people knowing their status quickly so that they don’t risk infecting others.
I’m also curious if the actually sensitivities of tests are known clearly after certain waiting periods. I saw something about some tests only being 50%-75% sensitive by day 7, which would seem too low to have confidence in results. But I have no idea if that’s a guesstimate or backed with real data.
Anyway, can you help unpack what’s going on here?
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H. Hunter Handsfield, MD
1 months ago
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Greetings. Thanks for an opportunity to explain this. It's an interesting issue that comes up on the forum from time to time. While I don't know the details of the particular tests you are concerned about, in general when a new diagnostic test is developed, the research is not designed to determine the time after exposure required to have a reliable result. Instead, a long interval is chosen in order to maximize test performance that may be stated when the test comes to market. Research to know the time to conclusive results would be difficult and expensive, and that kind of research and it isn't required by most countries' regulatory agencies (like the US Food and Drug Administration). When the test is approved for marketing, typically drug ads and other promotional approaches require a statement like "detects ___% of infections at least 2 weeks duration."
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Those research results do not mean the test cannot detect earlier infections, only that it has not been studied. Once the product is in use, labs doing the test -- and health care providers ordering the test -- gain experience in testing persons sooner than 2 weeks after exposure. Typically they also understand the known biology of the pathogen, e.g. how long it takes chlamydia or gonorrhea to grow to detectable levels. Such newer data may or may not be published in the scientific literature -- but even if published, usually the test producer is required to market the product based on the original data. Hence 2-3 weeks often is the stated time to conclusive results for a test that actually becomes positive sooner, typically 2-3 days for gonorrhea testing or 4-5 days for chamydia.
I can assure you that few if any STI experts -- including Dr. Hook and I -- rarely if ever encounter patients with gonorrhea or chlamydia in whom it took longer than 2-3 days (or 4-5 days) after exposure to test positive. However, keep two caveats in mind: if the tested person has taken active antibiotics between exposure and testing, a positive result may be delayed, at least in theory. Second, fewer data are available on other STIs, such Mycoplasma genitalium, which is inherently slow growing and might need longer after exposure for the lab test to turn positive. Finally, of course any patient is free to seek later testing if they have reason to doubt the reliability of their negative test result.
I hope this information is helpful. Let me know if anything isn't clear.
HHH, MD
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1 months ago
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Makes a lot of sense. Interesting!
On specific tests, I was curious about gonorrhea and chlamydia - so thanks for unpacking those.
What about trich? Is there less intel here because it’s parasitic rather than bacterial? What is your expert view on how many days it takes for NAAT tests to become conclusive? And does that differ for men vs women given it seems the parasite may replicate more slowly in men?
My other follow-up would be related to condoms. Similarly, you seem to have high confidence in their ability to prevent transmission of G / C / T… but much online guidance is much more conservative. I do realize repeated exposures vs single instance / occurrence is one lens that can shape the discrepancy in guidance… as well as perfect vs imperfect usage. But honestly, a lot of online sites would make you think condoms are “good not great”. Why is that? I’m guessing running tests with real people in clinical trials would be near impossible versus self reporting (which has accuracy issues).
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H. Hunter Handsfield, MD
1 months ago
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Apologies for the delay in this reply.
The several standard nucleic acid amplification tests (NAATs) for Trichomonas vaginalis have not been nearly as extensively studied as those for gonorrhea and chlamydia. Sorry that I don't have data available, but I would assume the results are reliable within a week, perhaps sooner.
You've hit the nail on the head in regard to interpreting advice on condom effectiveness. You nicely describe the differences between "biological effectiveness" and "use effectiveness." The former means a properly used condom that doesn't break or slip too much; in place before partial contact; and so on. The latter takes these uncertainties into account. Use effectiveness also relies on exposed persons memories and descriptions of proper use, which obviously can sometimes be very unreliable (think alcohol and other drugs). Finally, there are important differences for the various STIs. In particular, even consistent and proper use of condoms appears to provide little protection against HPV, undoubtedly because even the best use still allows substantial skin-skin contact above the condom.
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16 days ago
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I’ll ask some last academic-ish questions with my last reply.
Is there is any meaningful gap between the moment someone is infected with gonorrhea, chlamydia, and/or trich in their genitalia (penis or vagina) and when they are able to transmit that STD to another person through penis-in-vagina sexual intercourse?
Relatedly, does an infected person become more efficient at transmitting an STD over time because bacteria and or parasites increase? And if so, does this partially explain how communities with good access to testing keep STD rates lower (as they may catch an STD before peak infectiousness)?
Totally recognize there are probably limited to zero true studies on this. But maybe there’s some logic in how bacteria and parasites replicate that gives directional insights or thoughts.
Thanks for the explanations. I find the topic quite interesting given how little is openly discussed / shared publicly at deeper levels for laypersons.
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H. Hunter Handsfield, MD
15 days ago
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The opportunity for follow-up comments is for clarification of the original question. The quick answers are that such infection can't be transmitted until the numbers of growing bacteria become sufficient -- a couple days for gonorrhea, maybe a week for chlamydia, and unknown for trich but I would guess a week or so. 2-3 weeks for syphilis. For most of these, people become less infectious for others over time, as the body's immune system controls and eventually eliminates the infection.
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That completes the two follow-up comments and replies included with each question and so ends this thread. I hope the discussion has been helpful. Best wishes and stay safe.