Creatinine is a natural by-product of the break down of muscle tissue in the body. It is created on a more or less constant basis by all persons, the rate depending directly on the amount of muscle mass they have. It is filtered out of the body by the kidneys with a small amount passing into urine but the majority being used to create other molecules by normal bodily processes.
Since the rate of creation and filtering of creatinine is generally constant, a high level of creatinine in the blood is generally a good indicator of kidney function, with high levels indicating kidney dysfunction or failure. However, as an individual's "natural" creatinine level might vary from person to person, monitoring the trend of the creatinine level is a better diagnostic indicator than the absolute creatinine level.