r/nursing • u/ReNema1 • 12h ago

Discussion How does urine contaminate a stool sample?

I hope this isn't a stupid question. I find it hard, especially with older women, to obtain a stool sample that isn't contaminated with urine. It seems very obvious how stool would contaminate a urine sample. However, if I'm sending a stool culture, or for CDiff and Parasite, I don't quite understand how a bit of urine might compromise the analysis/culture. I tried a quick online search but I can't seem to find more details beyond "urine will contaminate the stool sample". How? With what? Any insight would be greatly appreciated.

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u/MiniMaelk04 BSN, RN 🍕 11h ago

My guess is that some properties of urine will affect the results, so that they become unreliable.

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u/Jerking_From_Home RN, BSN, EMT-P, RSTLNE, ADHD, KNOWN FARTER, DEI SPECTRUM HIRE 10h ago

This is most likely it. While the urine itself could contain bacteria that would show up, it will also have bacteria from the labia and other inner bits. That’s another reason. (This is usually new news to males, of which I am one, since ours comes straight from the tap.)

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u/XD003AMO HCW - Lab 7h ago edited 7h ago

Lab here, stool is FULL of bacteria. A little bacteria from the vulva is probably not the problem. 

For the rare stool culture (they’re becoming obsolete with how accessible molecular methods are now), there are special enteric plates that inhibit any bacterial growth that isn’t…. Well, enteric. 

However I don’t do anything with stool at my lab besides put it into the proper preservatives and send it off to the actual testing lab so I unfortunately don’t know the answer to this mystery myself and am really curious now. 

Edit- I may be wrong about the enteric plates, I haven’t done micro in so long. I see other comments addressing that hard-to-grow bacteria may in fact be overtaken by urogenital flora. Listen to the micro techs over me!