Creatinine

This test measures the amount of creatinine in your blood and/or urine. Creatinine is a waste product produced in your muscles from the breakdown of a compound called creatine. Creatine is part of the cycle that produces energy needed to contract your muscles. Both creatine and creatinine are produced by the body at a relatively constant rate. Almost all creatinine is excreted by the kidneys, so blood levels are a good measure of how well your kidneys are working. The quantity produced depends on the size of the person and their muscle mass. For this reason, creatinine concentrations will be slightly higher in men than in women and children.

Results from a blood creatinine test and a 24-hour urine creatinine test may be used to calculate creatinine clearance.

A blood sample is drawn from a vein in the arm. You may be asked to collect a complete 24-hour urine sample in addition to having your blood drawn. Your doctor or the laboratory will give you a large container and instructions for properly collecting this sample. Typically, you start collecting urine after you wake up in the morning and empty your bladder. This morning urine sample is not collected, but the time is recorded and used as the start time for the 24-hour collection. The next time you empty your bladder, you save this urine and all of the urine produced until the same time the following day.

No test preparation is needed.