INSTRUCTIONS FOR COLLECTING A 24-HR. URINE SPECIMEN
I spend 2-3 minutes explaining in depth how to collect the specimen.
First, I identify a day in which they can stay around the house, provided the lab is open the following day to receive the specimen. Then I inquire what time they usually wake up in the morning. Let’s say it’s 7:00 AM. I explain:
1. “So at 7:00 we start the clock for the 24-hours. That means you need to empty the bladder & kidneys of all the urine that’s been sitting around overnight. So go to the bathroom, and urinate into the toilet, NOT the container. Write the time down, like ‘7:05 AM’.
2. “From then on, all day & all night, every time you have to go, urinate every drop into the container. Store the container at room temperature or in the refrigerator [don’t worry; urine doesn’t have germs in it]. If you have to move your bowels, try real hard to keep the urine separate.
3. “When you wake up the next morning, either 10 minutes before or after the ‘7:05’ or whatever time you’d written, end the collection by urinating into the container. If you have to go an hour before, drink lots of fluids so you can go again. If you only have 15-20 minutes, try real hard to wait until the time.
4. Bring the urine in to the lab that day.
5. Remember, to begin the 24-hours, unirnate in the toilet, NOT the container. The next day, to end the collection time, urinate in the container.
INTERPRETING THE 24-HOUR URINALYSIS
The best way to determine if there’s been over- or under-collection is to simultaneous measure 24-hr. urinary creatinine. Men under 50 excrete 20-25 mg/kg; women 15-20. From age 50, values progressively decline, until at 90 y.o. the expected amount of creatinine will be only half that of younger persons. People with lots of muscle mass excrete more creatinine, the emaciated or cachectic less.
The volume of collection is key to determining if there was incorrect collection. An average person excretes around 80 cc. of urine per hour, which equals around 2,000 cc. per 24-hr. period. So a total volume around 3,000 cc. suggests over-collection (they probably began the time period by voiding into the container, instead of discarding it). A volume close to 1,000 cc equals under-collection (maybe they missed a number of voids, or they urinated into the toilet to both start and end the time period).
In such cases, all we can do is try to extrapolate from the results. Or we could give up & simply order a spot urine for albumin-to-creatinine ratio, knowing it won’t be as exact. We’d especially be wary in the body-builder with large muscle mass (underestimation of proteinuria due to high creatinine in the ratio’s denominator), or in the frail or undernourished (overestimation of proteinuria).