Creatinine is a waste product formed from the breakdown of creatine phosphate in muscle tissue. It is produced at a relatively constant rate proportional to muscle mass and is almost entirely filtered by the kidneys and excreted in urine without significant reabsorption or secretion. This makes creatinine a useful and stable marker of kidney filtration function.

Rising creatinine indicates declining kidney function — but it is a relatively late marker, as the kidneys need to lose approximately 50% of their function before creatinine rises noticeably. Creatinine also varies with muscle mass: athletes and muscular individuals may have higher creatinine that appears outside the normal range despite healthy kidney function, while elderly or frail individuals may have low creatinine masking poor kidney function. eGFR, calculated from creatinine, is more useful for staging kidney disease.