The 24-Hour Urine Collection Process

You must collect every drop of your urine during the 24-hour collection period. The volume of the urine is not important as long as it represents every drop that you pass. If you feel that the volume of urine will be greater than can be contained in the collection bottle provided by the lab, ask them for a second bottle. If you need to have a bowel movement, you must separate the urine in time or space. If you are asked to collect multiple 24 hour collections, you must keep each days collection separate.
Begin the collection at the usual time that you awaken; the preferred time is about 8 AM.
At that time, pass your urine, flush it down the toilet and note the exact time. You will now have an empty bladder and an empty collection bottle. That starts the collection.
Collect every drop during that day and night and finish the collection by voiding at exactly the same time the next morning; this last specimen should be added to the bottle.
The time of the final urine specimen should vary by no more than five or 10 minutes from the time of starting the collection the previous morning. If you have to urinate one hour before the appointed time, drink a full glass of water or more so that you can void again at the appropriate time. If you have to urinate 20 minutes before, hold the urine until the proper time. This is essential to the proper interpretation of the test results.

You will have finished the 24 hour urine collection at this point and the collection bottle should be returned to the laboratory.
The bottle(s) may be kept at normal room temperature for a day or two, but should be kept cool or refrigerated for longer periods of time. The bottle contains a weak acid. If you get acid or urine from the bottle on your skin or clothing, rinse your skin or clothing immediately with plenty of cold water, and you will have no problem.
The laboratory will usually measure a substance called creatinine in your urine. It will tell us whether each urine collection is complete. Unfortunately, that is all that it can tell us because it is excreted more or less constantly during the day, whereas the rate of excretion of other substances, such as adrenal steroids, may vary markedly. Therefore, loss of two hours of urine at any time of day or night will cause the same loss of creatinine, but the loss of other substances may be great or small. Consequently, we cannot correct the result of an improper collection. We can only recognize that it is improper and disregard the result. This may mean that we have to repeat the test, which can take as long as three additional
days.
Proper diagnosis and decision as to appropriate treatment depend entirely upon proper test results. It is therefore important that you do everything you can to provide a complete collection.