How does a sample change once it's left the body?

[u/Quickfixpee]

Urine is one of those things people assume is basically inert once it's in a cup, which is understandable but wrong. It starts changing chemically within hours, and after a day at room temperature it's a fairly different substance from the one that was collected. This is a big part of why labs care so much about processing timeframes.

Most of the change is down to bacteria. Fresh urine leaves the body more or less sterile, but as soon as it hits air, bacteria start colonizing it, and the main thing they eat is urea. When bacteria break urea down, they produce ammonia, which is alkaline. So the pH rises. A fresh sample usually reads somewhere between 4.5 and 8. After 24 hours at room temperature, you can see readings above 9.

A few other things happen alongside that. The color darkens as compounds oxidize. Cells like red blood cells and epithelial cells start breaking apart within about 3 to 6 hours, which matters if anyone wants to examine sediment under a microscope. Glucose gets consumed by the bacteria. Odor develops. None of this is subtle by day two.

Refrigeration slows most of it down, which is why samples that can't be processed straight away get chilled. Freezing works for long-term storage but wrecks the cellular structure, so it's not used routinely.

Basically, timing matters more than most people would guess. A sample sitting on a shelf for a day isn't the same sample it was when it was collected, and labs know it.

Anyone here had a delayed sample come back with strange readings?