When I was hospitalized last year with hypertension issues, one test I underwent involved saving an entire days worth of urine. Which was carefully kept on ice.
My question is why? What difference does keeping it cold make to measuring for sodium, if any? I mean it’s always warm when it’s in your body, so it’s already way cooler once it’s outside your body, surely.
I think that is to prevent the growth of bacteria and other microorganisms that could affect the results of various tests For example, the microbes could metabolism sugar, change the pH, etc. I’m not sure if it would affect the sodium test, but I suppose they have one collection protocol, independent of the specific test they’ve ordered.
Are you sure it was for measuring sodium? You’re correct that urinary sodium is pretty stable and would not need to be kept on ice/cold.
However there are other tests sometimes performed to look for hormonal issues that can cause hypertension and it would make more sense to keep the sample cold if that was what they were looking for. In around 5-10 % of people with hypertension it’s caused by an overproduction of one of a few different sets of hormones (epenephrine & related hormones, cortisol & aldosterone). This is more likely in patients who are relatively young and/or have hypertension which is severe or difficult to control with medication.
Since you were hospitalised it seems likely your hypertension was fairly severe so they may well have been testing for one of these conditions and epenephrine & cortisol are both commonly measured in 24 hour urine collections and are not as stable as sodium - they can be metabolized by bacteria and degrade through other chemical reactions. My local hospital protocols mandate an acid preservative for the epenephrine but it’s conceivable your hospital protocols could include keeping the sample cold.