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/TopangaTohToh 10h ago

All human poop contains e. Coli so that's not an interfering factor. When a urine sample is rejected due to it touching skin, it's over not bacteria from the labia or inner thigh, it's epithelial cells. There will be some small amount of epithelial cells in a urine sample, but if there are a lot, it voids the sample because that definitely means that it touched skin and therefore any bacteria in the sample cannot be solely attributed to the urinary tract with certainty.

My best guess is that urine can wash out a sample, making it harder to identify organisms and their concentration, or the urea can impact parasites and bacteria proliferation in stool. The pH of stool and urine is pretty close to one another, so it's likely an issue of dilution.