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).

Physiology

It is mainly filtered by the kidney, though a small amount is actively secreted. Creatinine is not reabsorbed. If the filtering of the kidney is deficient, blood levels rise. This effect is used as an indicator of renal function. However, in cases of severe renal dysfunction, the creatinine clearance rate will be overestimated because the active secretion of creatinine will account for a larger fraction of the total creatinine cleared. Men tend to have higher levels of creatinine because they have more skeletal muscle than women.

Diagnostic use

Measuring serum creatinine is a simple test and it is the most commonly used indicator of renal function. A 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.

Interpretation

In the United States, creatinine is typically reported in mg/dL, while in Europe μmol/liter is used. 1 mg/dL of creatinine is 77.2 μmol/l.

The reference range for women is considered 0.5 to 1.0 mg/dL (about 40-80 μmol/l), while the reference range for men is 0.9 to 1.4 mg/dL (70-110 μmol/l). While a baseline serum creatinine of 2.0 mg/dL (150 μmol/l) may indicate normal kidney function in a male body builder, a serum creatinine of 0.7 mg/dL (60 μmol/l) can indicate significant renal disease in a frail old woman. More important than absolute creatinine level is the trend of serum creatinine levels over time. A rising creatinine indicates kidney damage, while a declining creatinine level indicates improving kidney function.