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Creatinine (from the Greek κρέας, 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 muscle mass). In chemical terms, creatinine is a spontaneously formed cyclic derivative of creatine. Creatinine is chiefly filtered out of the blood by the kidneys (glomerular filtration and proximal tubular secretion). There is little-to-no tubular reabsorption of creatinine. If the filtering of the kidney is deficient, creatinine 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 blood urea nitrogen 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, in general, have more skeletal muscle mass than women. Vegetarians have been shown to have lower creatinine levels.
There exists several tautomers of creatinine. Ordered by contribution, they are:
- 2-Amino-1-methyl-1H-imidazol-4-ol or 2-Amino-1-methylimidazol-4-ol
- 2-Imino-1-methyl-2,3-dihydro-1H-imidazol-4-ol or 2-Imino-1-methyl-3H-imidazol-4-ol
- 2-Imino-1-methyl-2,5-dihydro-1H-imidazol-4-ol or 2-Imino-1-methyl-5H-imidazol-4-ol
Measuring serum creatinine is a simple test and it is the most commonly used indicator of renal function.
A rise in blood creatinine level 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 (CrCl) test. Creatinine clearance can be accurately calculated using serum creatinine concentration and some or all of the following variables: sex, 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, sex, and weight are included in the patient information.
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