Creatinine

Creatinine is a breakdown product of creatine phosphate in muscle, and is usually produced at a fairly constant rate by the body (depending on muscle mass). It is filtered by the kidney. If the filtering of the kidney is deficient, blood levels rise. This effect is used as an indicator of renal function.
Measuring serum creatinine is a simple test and it is the most commonly used indicator of renal function. It should be remembered that rise in blood creatinine levels is observed only with marked damage to functioning nephrons; therefore this test is not suitable for detecting early kidney disease. A better estimation of kidney function is given by the creatinine clearance test, which, however, is less convenient and unsuitable as a screening test because it requires the collection of urine over 24 hours.

Reference range

The reference range for creatinine is 50 to 110 μmol/L (0.8 - 1.4 mg/dl).
Men tend to have higher levels of creatinine because they have more skeletal muscle than women.
Serum creatinine is not affected by diet or activity level.