It is generally accepted that creatinine produced by endogenous metabolism is derived from muscle creatine and phosphocreatine. The conversion is apparently the result of an irreversible process of normal metabolism which takes place at a constant rate, proportional to muscle mass and independent of muscular exercise.*
The daily urinary excretion of creatinine is constant for the individual, ranging from 1.5 to 2.0 gm. for men and from 0.8 to 1.5 gm. for women. This corresponds to approximately 2% of the total body creatine, from which it is derived. This excretion rate is apparently independent of protein ingestion and is considered an index of muscle metabolism. It is not influenced by exercise or urine volume. Decreased creatinine excretion, with concurrent elevation in plasma level, is generally indicative of impaired renal function, since creatinine is freely filterable at the glomerulus. Decreased excretion in the absence of elevated plasma concentration is usually due