Creatinine (from the Greek ''kreas'', flesh) is a break-down product of creatine phosphate in muscle, and is
usually produced at a fairly constant rate by the body (depending on
Chemically, creatinine is a spontaneously
formed cyclic derivative of creatine.
Creatinine is chiefly filtered out of the blood by the kidneys, though a
small amount is actively secreted by the kidneys into the urine. There
is little-to-no tubular reabsorption of creatinine. If the filtering of the kidney is deficient, blood levels rise. Therefore, creatinine levels
in blood and urine may be used to calculate the creatinine clearance
(CrCl), which reflects the glomerular filtration rate (GFR). The GFR is clinically important because it is a measurement 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. Ketoacids, cimetidine and trimethoprim reduce creatinine
tubular secretion and therefore increase the accuracy of the GFR estimate, particularly in severe renal dysfunction. (In the absence of
secretion, creatinine behaves like inulin.)
A more complete estimation of renal function can be made when
interpreting the blood (plasma) concentration of creatinine along with that of urea. BUN-to-creatinine ratio (the ratio of urea to
creatinine) can indicate other problems besides those intrinsic to the kidney; for example, a urea level raised out of proportion to the
creatinine may indicate a pre-renal problem such as volume depletion.
Men tend to have higher levels of creatinine because they generally
have more skeletal muscle mass than women. Vegetarians have been
shown to have lower creatinine levels.
Creatinine Diagnostic Use
Plasma creatinine (PCr)
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 stage kidney disease. A better estimation of kidney function is given by the creatinine clearance test. Creatinine clearance can be accurately calculated using serum creatinine concentration and some or all of the following variables: ***, age, weight, and race as suggested by the American Diabetes Association without a 24 hour urine collection. Some laboratories will calculate the CrCl if written on the pathology request form; and, the necessary age, ***, and weight are included in the patient information.
A recent Japanese study suggests that a lower serum creatinine level is associated with an increased risk for the development of type 2 diabetes in Japanese men.
Urine creatinine (UCr)
Creatinine concentration is also checked during standard urine drug tests. High creatinine levels indicate a pure test while low amounts of creatinine in the urine indicate a manipulated test, either through the addition of water in the sample or by drinking excessive amounts of water
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