This test measures the amount of creatinine in the blood and/or urine. Creatinine is a waste product produced by muscles from the breakdown of a compound called creatine. 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. Almost all creatinine is excreted by the kidneys, so blood levels are usually a good indicator of how well the kidneys are working.