Creatinine is present in all body secretions and is a by-product of muscle metabolism, formed by a spontaneous and irreversible conversion from creatine and creatine phosphate. The formation of creatinine is proportional to total muscle mass, and approximately to body weight. The production rate shows minimal daily variations, unless the muscle mass changes, with 2% of whole body creatinine being transformed every 24 hours. Since creatinine is excreted at a relatively constant rate, measurement of urinary creatinine can indicate if the urinary concentration has been adjusted by in vivo or in vitro dilution.