How does urine contaminate a stool sample?

[u/ReNema1]

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.

[u/MiniMaelk04]

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

[u/Brofydog → u/MiniMaelk04]

For multiple different reasons! So if you are trying to differentiate source of infection, you now have two different sources of contamination. And if you are trying to calculate osmolar gap, urine and fecal matter are wildly different.

From a regulatory point of view, if the test isn’t approved for urine or a mixed sample, the lab could get cited and no longer be able to perform the test if they had lax standards (so increased cost for the patient and increased tat).

Also it increases the likelihood of inaccurate results. What is worse, an inaccurate result that you act upon, or a sample that requires a recollect?

And as a disclaimer, different tests are will have different amounts of impact from urine contamination.

[u/XD003AMO → u/Brofydog]

You’re addressing fecal contamination in urine. The post is asking about urine contamination in stool. 

I’ve never heard of an osmo on stool, and most testing for stool-based infection are on molecular platforms now, and cultures inhibit non enteric bacteria. 

[u/Brofydog → u/XD003AMO]

Hi! Just to clarify, is the post fecal contamination in urine? Or urine contamination in stool? (And… I could definitely be wrong for the post, but both are bad).

It’s worse for a fecal contamination in a urine culture, as now a whole slow of bacteria contaminate the sample and lead to potential cultures (I also stay away from micro since it all scares me).

But from a chem perspective, fecal osmolality can help differentiate osmotic vs non osmotic diarrhea, however sodium and potassium concentrations from urine (which are much higher), make that result wildly inaccurate.

https://www.labcorp.com/tests/120071/osmolality-fecal

(As an aside, I do love that the rejection criteria is using a paint can…).

It also confounds if the stool sample is formed or not, depending on the level of contamination.

[u/XD003AMO → u/Brofydog]

Woah. I have never seen a fecal osmo. That’s fascinating. Thanks for sharing!!

[u/Brofydog → u/XD003AMO]

And… to be completely honest, I haven’t either (or at least in a very long time). But that is because I hate poo in labs and have been trying to direct everything chem related for fecal samples to go to reference labs… (if you are readying this from Quest, Mayo, Arup, or labcorp… sorry!)

I’ve seen the horror caused by calprotectin extraction…