Sometimes creatinine levels are tested over a 24 hour period. You’ll be asked to collect all of your urine over that time frame and bring it in to your doctor. The lab will test the amount of creatinine in your urine, and then compare that to the amount of creatinine in your blood. This shows your doctor how much waste is being filtered out of your body and how your kidneys are performing.
Sometimes your care team needs answers faster. If you’re experiencing acute kidney failure, it’s not effective to take a whole day to collect urine. Plus, most doctors don’t want to put you through the inconvenience of collecting urine for a whole day. That’s when doctors use a simple blood test and a formula to estimate your creatinine clearance.