An anhydride of creatine, creatinine is formed and excreted in constant amounts by an irreversible reaction and functions solely as the main end product of creatine. Creatinine production is proportional to total muscle mass and is relatively unaffected by urine volume or normal physical activity or diet.
An excellent diagnostic indicator of renal function, the creatinine clearance test determines how efficiently the kidneys are clearing creatinine from the blood. The rate of clearance is expressed in terms of the volume of blood (in milliliters) that can be cleared of creatinine in 1 minute. Creatinine levels become abnormal when more than 50% of the nephrons have been damaged.
Procedure and posttest care
Reference valuesNormal creatinine clearance varies with age; in males, it ranges from 55 to 146 ml/minute/1.73 m2; in females, from 52 to 134 ml/minute/1.73m2. For older patients, creatinine clearance normally decreases by 6 ml/minute for each decade.
Low creatinine clearance may result from reduced renal blood flow (associated with shock or renal artery obstruction), acute tubular necrosis, acute or chronic glomerulonephritis, advanced bilateral chronic pyelonephritis, advanced bilateral renal lesions (which may occur in polycystic kidney disease, renal tuberculosis, and cancer), nephrosclerosis, heart failure, or severe dehydration.
High creatinine clearance rates generally have little diagnostic significance.
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