Creatinine is a waste product derived from endogenous sources by tissue creatine breakdown and is excreted by the kidney. Creatinine is the anhydride of creatine. The reaction occurs nonenzymatically. About 98% of the total creatine is in the muscle as creatine phosphate. About 2% of the total creatine is converted daily to creatinine so that the amount of creatinine produced is related to the total muscle mass. Creatinine coefficient is more precise and is used to assess the functional muscle mass in the body. The excretion of creatinine increases in fevers and wasting diseases.