Creatinine is the end-product of creatine metabolism.  The  presence  of  creatine  is  primarily  in  muscle.  The amount of creatinine is related to total skeletal muscle mass. Daily creatinine production is regular, except when there is a massive injury to muscle.  In healthy individuals, the kidneys excrete creatinine, and blood levels and daily urinary excretion of creatinine fluctuate very little.