Article: Creatinine

Chemical structure of 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).

Physiology

It is mainly filtered by the kidney, though a small amount is actively secreted. There is some tubular reabsorption of creatinine, but this is compensated by a roughly equivalent degree of tubular secretion. 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. A higher than normal creatinine and BUN can also be indicative of dehydration when BUN-to-creatinine ratio is abnormal, with BUN levels rising higher than creatinine levels. 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. Creatinine Clearance can accurately be calculated using serum creatinine concentration and some or all of the following variables: sex, age, weight, and race as suggested by the National Diabetes Association without a 24 hour urine collection ^[1]. Some laboratories will calculate the ClCr if written on the prescription; and, the necessary age, sex, and weight are included in the patient information.

Interpretation

In the United States, creatinine is typically reported in 0.7 - 1.5 mg/dL, while in Canada and Europe μmol/litre may be used. 1 mg/dL of creatinine is 88.4 μmol/l.

The typical reference range for women is considered 0.5 to 1.0 mg/dL (about 45-90 μmol/l), for men 0.7 to 1.2 mg/dL (60-110 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 level indicates kidney damage, while a declining creatinine level indicates improving kidney function.

Sports nutrition

Supplemental creatine is often used by bodybuilders and other athletes to increase muscle size and endurance.