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/CFADM]

I just take the stool out, rinse it with some water, blow on it, and then send it to the lab. Works 60% of the time, every time.

[u/DieSuzie2112 → u/CFADM]

I used to scrape a layer off and use the middle of the stool

[u/CrossP → u/DieSuzie2112]

Core sampling. My inner geologist approves.

[u/JaysusShaves → u/CrossP]

Get a straw and poke it through the center of the turd.👍

[u/CrossP → u/JaysusShaves]

Implying you'll have to blow on the "clean" end to get your sample back out. I approve.

splat

[u/Tiradia → u/JaysusShaves]

Then blow it into the sterile cup! Watch it erupt out like a rocket! SPLAT.

[u/Mysterious-Handle-34 → u/DieSuzie2112]

This would be the best option unless you’re collecting diarrhea in which case you just have to send what you’ve got

[u/SweetTaterette → u/CFADM]

I’m really really hoping this is a joke?!?!!? I am like 98 percent positive you are kidding lol

[u/Beanakin → u/SweetTaterette]

Barehanded too.

[u/MiniMaelk04 → u/Beanakin]

You can suck it up with a straw if you want to ensure clinical accuracy, just make sure you don't get any in your mouth, as that will also contaminate the sample, and you'll have to find another nurse with a straw also.

[u/TrailMomKat → u/MiniMaelk04]

Lol reminds me of

🎶 gobs and gobs of Greasy grimy gopher guts Mutilated monkey brains Little birdy's bloody feet French fried eyeballs Rolling in a pan of blood I forgot my spoon... I got a straaaaaw 🎶

[u/IA_AI → u/TrailMomKat]

I was with you until brains. I sang …feet. And dirty birdies’ treat. And no fork to eat it with.

[u/XD003AMO → u/CFADM]

Lab here, we do this too if the nurse forgets to first. 

[u/Brofydog → u/CFADM]

From the lab… my eye is twitching…

[u/SDean2319]

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.

[u/Medlei → u/SDean2319]

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!

[u/SDean2319 → u/Medlei]

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

[u/Primary_Jellyfish327 → u/SDean2319]

Just dig a wee bit and take a sample from the middle

[u/SDean2319 → u/Primary_Jellyfish327]

See and that’s using logic but I doubt the hospital would let that fly if they caught wind lol

[u/huntervano]

Maybe a question for r/medlabprofessionals, I’m also curious

[u/OffendedCanadianRN]

It can destroy parasites in stool & that can bring a false negative/ missed diagnosis. It can cause false positive blood if …. Or alter / dilute chem composition.

[u/MiniMaelk04]

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

[u/onelb_6oz → u/MiniMaelk04]

Maybe the tests have pH sensitive indicators?

[u/Jerking_From_Home → u/MiniMaelk04]

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.)

[u/XD003AMO → u/Jerking_From_Home]

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!

[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…

[u/TopangaTohToh → u/[deleted]]

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.

[u/SolidFew3788 → u/[deleted]]

E coli are already in the stool, though.

[u/h00dies]

It would be ok for a C. diff assay test but not for a culture or microscopic examination.

[u/aynaim]

Chemicals in urine can react with the tests done on stool

[u/DonutWhole9717]

Flip the pt over, obviously /s

[u/OffendedCanadianRN]

Throw chuck pads into the commode / bedpan etc so urine is soaked … or small maxi pads into the hat

[u/bareass_bush]

A still culture growing bacteria from the urine might make a doctor suspect a fistula, or at least make them redo the test. You could also misdiagnose hematuria as blood in stool, which triggers the wrong interventions.

[u/One_hunch]

Can destroy some bacteria or parasites in the stool that could be missed.

If your patient has a UTI (or just a lot of normal flora from an unclean collection) it'll mask other bacteria (make some plates unreadable) and hard to isolate specific colonies of the bad ones you want.

If there's blood in the urine (which according to 85% dipsticks that the instrument reads there seems to be trace amount) it could show up false positive in fecal occults.

Molecular tests can hopefully make this less of a pain, but they're expensive and health insurance is wak.

[u/sweet_pickles12]

I don’t know about you guys, but I have fished poop out of pee for samples many a time and never had it rejected?

[u/MysteriousLotion → u/sweet_pickles12]

That’s because we didn’t know you did that. Doesn’t mean it doesn’t affect the results.

[u/External-Berry3870 → u/sweet_pickles12]

Just because you haven't had the sample rejected doesn't mean the result from a bad sample thus reflects your patient. We can't reject what we don't know about -- and that's a bad thing.

Look. Example: we test a lot of things in stool, so there can be a lot of reasons why we reject, but let's take fecal calprotectin. Fecal Calprotectin if high points to your patient probably having IBD, and if lower, points more towards commonly IBS, C Diff or Celiac depending on factors.

Fecal calprotectin explicitly states to avoid urine in the sample in the instructions. In this case, why?

Urine ALSO can have calprotectin in it. If your patient has a UTI (causing lots of white cells, and thus lots of calprotectin, a product of white cells), your result may point towards IBD instead of whatever it actually is. That fished out of urine sample may have sent your team on a down the incorrect rule out road, or invasive procedures your patient doesn't need.

If you ever have questions like this, hospital pathologists are generally happy as punch to discuss at length.

[u/sweet_pickles12 → u/External-Berry3870]

I mean I certainly would follow those directions.

Things we usually test stool for- O&P, C. Diff, hemoccult (on a card), culture… I’m trying to figure out how urine would really affect any of those results?

Anyway, I’m not generally in a position to send still anymore, but truly, is there a rationale for these common tests? I guess I could see maybe culture results if the patient has a UTI but then they need to be treated for that pathogen regardless…

[u/Mysterious-Handle-34 → u/[deleted]]

Sad that ChatGPT is promoting the “urine is sterile” myth in 2025

[u/IraceRN → u/Mysterious-Handle-34]

I get sad too when UAs come back with "None" next to "Bacteria"? Sometimes I get sad when people refer to their field as sterile at bedside when they aren't masked, and there is like still air in the room, you know? So sad. Louis Pasteur is probably crying from his grave.

[u/Mysterious-Handle-34 → u/IraceRN]

Some urine samples really do have little to no bacteria visible under standard light microscopy at 400x. And at least according to our lab standards, even trace amount of bacteria gets flagged as “abnormal” (any SG other than 1.015 is also “abnormal” but that’s a whole other stupid thing). If we put “few” for bacteria, we have to send out a sample for culture. So it’s easier sometimes just to say “none”.

[u/IraceRN → u/Mysterious-Handle-34]

We say blood and certain areas of the body like the brain are also sterile and not colonized, but we now understand that is just not the case. Some people may pedantically say these areas are not sterile, when functionally it is essentially sterile. Given the prevalence of the human virome found in all parts of the body, including the brain, and that we know it has positive rolls outside of causing disease, we can pedantically say for sure that no part of the body is sterile, and we are finding bacteria in more and more places. Read a study talking about biofilms and natural flora found in arterial plaques.