Creatinine is present in all body secretions and is a by-product of muscle metabolism, formed by the spontaneous and irreversible conversion of creatine and creatine phosphate. The formation of creatinine is proportional to total muscle mass and 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 by the kidneys, measurement of urinary creatinine can indicate if the urinary concentration has been adjusted by in vivo or in vitro dilution. The level of creatinine in the blood increases as kidney disease progresses.Synonyms: 2-amino-1-methyl-5H-imidazol-4-one