Creatinine is a chemical waste molecule that is generated from muscle metabolism. Creatinine is produced from creatine, a molecule of major importance for energy production in muscles. Approximately 2% of the body’s creatine is converted to creatinine every day. Creatinine is transported through the bloodstream to the kidneys. The kidneys filter out most of the creatinine and dispose of it in the urine.
Because the muscle mass in the body is relatively constant from day to day, the creatinine production normally remains essentially unchanged on a daily basis.
The kidneys maintain the blood creatinine in a normal range. Creatinine has been found to be a fairly reliable indicator of kidney function. Elevated creatinine level signifies impaired kidney function or kidney disease.
As the kidneys become impaired for any reason, the creatinine level in the blood will rise due to poor clearance of creatinine by the kidneys. Abnormally high levels of creatinine thus warn of possible malfunction or failure of the kidneys. It is for this reason that standard blood tests routinely check the amount of creatinine in the blood.
A more precise measure of the kidney function can be estimated by calculating how much creatinine is cleared from the body by the kidneys. This is referred to as creatinine clearance and it estimates the rate of filtration by kidneys (glomerular filtration rate, or GFR). The creatinine clearance can be measured in two ways. It can be calculated (estimated) by a formula using serum (blood) creatinine level, patient’s weight, and age. The formula is 140 minus the patient’s age in years times their weight in kilograms (times 0.85 for women), divided by 72 times the serum creatinine level in mg/dL. Creatinine clearance can also be more directly measured by collecting a 24-hour urine sample and then drawing a blood sample. The creatinine levels in both urine and blood are determined and compared. Normal creatinine clearance for healthy women is 88-128 mL/min. and 97 to 137 mL/min. in males (normal levels may vary slightly between labs).
Blood urea nitrogen (BUN) level is another indicator of kidney function. Urea is also a metabolic byproduct which can build up if kidney function is impaired. The BUN-to-creatinine ratio generally provides more precise information about kidney function and its possible underlying cause compared with creatinine level alone. BUN also increases with dehydration.