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There is a seemingly unfounded mantra that a person's urine is sterile who is not suffering from a UTI.

So far I have found convincing evidence to the contrary; urine is not sterile. This poster abstract and publication by Hilt et al., 2014 shows that urine may not be sterile. They list 6 other articles (references 3-8) that also found evidence for microflora in healthy urine, the urethra, and the bladder. In some of those studies males were used, and some used females. This makes a convincing case for non-sterile urine.

But why does this mantra persist; where does it come from? Is there a paper that demonstrates the sterility of urine that I have missed? Even if it is old literature, or pop culture, I'd appreciate it.

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    $\begingroup$ You answered your own question. Hilt et al's paper and others have had more than convincing evidence that urine is not sterile. As to why this idea persists? Overall, because of ignorance: either people don't put in the effort to learn real facts or (in the case of other biologists) they don't keep up with the literature. You also can't ask any biologist (e.g., a cell biologist or ecologist) if they believe urine is sterile and believe they know much (any) more than a non-biologist $\endgroup$ – theforestecologist Jan 28 '16 at 3:48
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    $\begingroup$ As there definitely are uropathogens, you can at least be sure urine is not always sterile. I have heard people talking about obligate pathogens in the urine and they definitely found microflora in healthy urine. But that was personal communication (and as far as I know the group studying their own urine because they needed to know whats there before using it to grow other bacteria, so not a study setting to be published). $\endgroup$ – skymningen Jan 28 '16 at 9:30
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    $\begingroup$ I think the main point about the sterility myth is that is around for a long time. And it took the group, which finally found the bacteria quite a while to proof this. So to this point nobody was able to find bacteria, thus is was declared sterile. We could have know better, since we are populated with bacteria. $\endgroup$ – Chris Jan 28 '16 at 11:59
  • $\begingroup$ In a healthy person urine is sterile up until the point it leaves the bladder through the urethra. At least, that's what I thought. It depends on where in the system you are looking at it. After it leaves the kidneys it must be sterile. $\endgroup$ – AliceD Mar 24 '16 at 10:10
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    $\begingroup$ @Christiaan My last comment came across quite aggressive, apologies! I thought sterile bladder urine was the de-facto case too until these papers were pointed out to me, but I can't find anything (peer reviewed) to support urine sterility. $\endgroup$ – James Mar 24 '16 at 10:46
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But why does this mantra persist; where does it come from?

Behind this line of inquiry is a fascinating observation about the disconnect between the laboratory and the clinic. The basic answer, from my perspective, goes like this:

  1. It is doctors who talk with people about their urine, and any testing thereof;
  2. for doctors, urine is practically1 sterile; and
  3. among those who are aware of the microbiologic reality, doctors inevitably gloss details when explaining laboratory results to patients.

The average clinician learns most of what he knows about his patients' urine from urinalyses. Most clinical urinalysis assays show negative nitrites and negative leukocyte esterase in normal people. A glance at the reference ranges for a clinical assay will demonstrate this. (See also the clinical review below.)

Ideally, urinalyses should not be performed in patients without relevant symptoms, but they are sent anyway, and they're mostly negative.2 Even in people who have symptoms possibly suggesting a UTI they're frequently negative. Furthermore, urine cultures from properly collected specimens grown on the media used in clinical laboratories mostly return at 72 hours: "no growth". This point is emphasized in the abstract by Hilt et. al. linked in the question. Of the cultures that grew in their "expanded quantitative urine culture" protocol,

90% of these specimens were deemed "No Growth" by the standard urine culture technique, highlighting its limitations.

The last phrase "highlighting its limitations" is an interesting one. It appears to me to be yet to be proven that this is a clinically meaningful "limitation" of such assays. Without attempting to refute these data — fortunately this question doesn't require me to do so — I will note that the two linked studies aimed to show that people with overactive bladder are characterized by increased urinary flora. The explicitly stated hypothesis is that this may contribute to symptoms. Even in this study demonstrating the existence of such a micro-biome even in normal people, then, it is understood as a potentially pathologic state.

The reason this "mantra persists", then, is that it remains true for clinical purposes. In contrast to most (all?) other bodily discharges, urine is not packed with bacteria, and the commonly used assays reflect this fact. Medicine and laboratory science have different "modes of discourse", each calibrated to convey levels of precision that are appropriate to the outcomes of interest.


Notes

1. In American English at least, that adverb is ambiguous, meaning either "for practical purposes" or (idiomatically) "almost". Both senses are intended here.

2. Of course, there are plenty of data in various populations demonstrating that some substantial minority of urinalyses among asymptomatic people return positive, "proving" that we shouldn't be checking them (amen!). But this is not the point. The gestalt remains: normal = negative.

3. The nitrite parameter in particular is not especially sensitive. It is, however, quite specific (92-100%), meaning it reliably returns negative in people without infection. (See review, below.) Of course, negative nitrites does not mean sterile in the sense that the OP has used it; see "modes of discourse", above.


Reference
Simerville JA, et al. Urinalysis: A Comprehensive Review. Am Fam Physician. 2005 Mar;71(6):1153-1162.

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  • $\begingroup$ Really great, satisfying, answer. $\endgroup$ – James Apr 20 '16 at 3:40

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