Creatinine is a waste product formed by the breakdown of a substance (creatine) important for converting food into energy (metabolism). The creatinine is filtered out of the blood by the kidneys and then passed out of the body in urine.
Creatinine is produced at a steady rate and is affected very little by diet or normal physical activities. If the kidneys are damaged and cannot function normally, the amount of creatinine in the urine decreases while the amount of creatinine in the blood increases.
WebMD Medical Reference from Healthwise