Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. Creatinine is removed from the body by the kidneys, which filter almost all of it from the blood and release it into the urine. This test measures the amount of creatinine in the blood and/or urine.
Creatine is part of the cycle that produces energy needed to contract muscles. Both creatine and creatinine are produced by the body at a relatively constant rate. Since almost all creatinine is filtered from the blood by the kidneys and released into the urine, blood levels are usually a good indicator of how well the 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 may be used in combination with results from other tests, such as a 24-hour urine creatinine test, to perform calculations that are used to evaluate kidney function. See the "How is it used?" section for more on these.