My two cents:

In my opinion, if you’re looking at sodium and potassium levels in the urine, in order to come to a conclusion about underlying medical condition, then you should look at 24-hour levels. I suspect that may be why you both only found ranges for 24-hour collections. I am not sure how informative a single reading would be, and perhaps that is why numbers are not available for a random test.

Also: Levels in the urine are expressed as a concentration, i.e. the total MASS of a substance found in a specified VOLUME of liquid. (MASS is expressed in units such as "mmol", while VOLUME is expressed in units such as "liter".) Thus, I also suspect that the numbers Megan gave indicate mmol/L/24h, rather than just mmol/24h. (There might have been a general footnote on the lab page that said something like all references given are for 1 liter of urine, or something like that.)