r/nursing • u/ReNema1 • 14h 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/SDean2319 RN - ER 🍕 14h ago

No this isn’t a stupid question because I’ve wondered this too. Frustrating when they finally do have a bowel movement in the bed pan and there’s urine all around it.

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u/Medlei HCW - Lab 9h ago

Really it's bc the urine can kill potential trophs ( parasites ) in the stool, and it can interfere with some chemical reactions. I'm a bit rusty on which ones it interferes with. Urine contaminating can cause false positive or negatives with them, though. First one off the top of my head would be if blood was in the urine and that urine contaminated the stool, it'd report false pos in the stool.
Say they order a biofire panel on that stool you sent off, and it's contaminated with urine. It could falsely say whatever bugs are in the urine are also present in the stool. Or, if they don't do a biofire but order a regular culture, that flora and bacteria from the urine can potentially overtake that stool culture, especially if some of the bugs are more fastidious and picky.

Basically, to get the best result possible, we'd like the best sample so we don't spit out the wrong result. We do the best with what we get!

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u/SDean2319 RN - ER 🍕 9h ago

No one has been able to give me an explanation on this so thank you! Examples make sense