<P> In clinical practice, however, creatinine clearance or estimates of creatinine clearance based on the serum creatinine level are used to measure GFR . Creatinine is produced naturally by the body (creatinine is a breakdown product of creatine phosphate, which is found in muscle). It is freely filtered by the glomerulus, but also actively secreted by the peritubular capillaries in very small amounts such that creatinine clearance overestimates actual GFR by 10% to 20% . This margin of error is acceptable, considering the ease with which creatinine clearance is measured . Unlike precise GFR measurements involving constant infusions of inulin, creatinine is already at a steady - state concentration in the blood, and so measuring creatinine clearance is much less cumbersome . However, creatinine estimates of GFR have their limitations . All of the estimating equations depend on a prediction of the 24 - hour creatinine excretion rate, which is a function of muscle mass which is quite variable . One of the equations, the Cockcroft and Gault equation (see below) does not correct for race . With a higher muscle mass, serum creatinine will be higher for any given rate of clearance . </P> <P> A common mistake made when just looking at serum creatinine is the failure to account for muscle mass . Hence, an older woman with a serum creatinine of 1.4 mg / dL may actually have a moderately severe degree of renal insufficiency, whereas a young muscular male can have a normal level of renal function at this serum creatinine level . Creatinine - based equations should be used with caution in cachectic patients and patients with cirrhosis . They often have very low muscle mass and a much lower creatinine excretion rate than predicted by the equations below, such that a cirrhotic patient with a serum creatinine of 0.9 mg / dL may have a moderately severe degree of renal insufficiency . </P> <P> One method of determining GFR from creatinine is to collect urine (usually for 24 h) to determine the amount of creatinine that was removed from the blood over a given time interval . If one removes 1440 mg in 24 h, this is equivalent to removing 1 mg / min . If the blood concentration is 0.01 mg / mL (1 mg / dL), then one can say that 100 mL / min of blood is being "cleared" of creatinine, since, to get 1 mg of creatinine, 100 mL of blood containing 0.01 mg / mL would need to have been cleared . </P> <P> Creatinine clearance (C) is calculated from the creatinine concentration in the collected urine sample (U), urine flow rate (Vdt), and the plasma concentration (P). Since the product of urine concentration and urine flow rate yields creatinine excretion rate, which is the rate of removal from the blood, creatinine clearance is calculated as removal rate per min (U × Vdt) divided by the plasma creatinine concentration . This is commonly represented mathematically as </P>

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