What is the creatinine clearance test? How can it be used to estimate GFR? How a

Online answer: "The creatinine clearance test compares the level of creatinine in urine with the creatinine level in the blood. (Creatinine is a breakdown product of creatine, which is an important part of muscle.) The test helps provide information on kidney function. The creatinine clearance test is used to estimate the glomerular filtration rate (GFR). However, because a small amount of creatinine is released by the filtering tubes in the kidneys, creatinine clearance is not exactly thto the same as the FR. In fact, creatinine clearance usually overestimates the GFR. This is particularly true in patients with advanced kidney failure.Factors that may interfere with the accuracy of the test are as follows:
Incomplete urine collection
Pregnancy
Vigorous exercise
Drugs that can interfere with creatinine clearance measurements include: cimetidine, trimethoprim, and drugs that can damage the kidneys, such as cephalosporins.

The creatinine clearance test should only be done for patients who are medically stable. Such patients may have a rapidly changing creatinine clearance, and therefore any result may be inaccurate."

This test relies on measuring total creatinine excreted in the urine in 24 hours and measuring the plasma concentration of creatinine. Since essentially none of the filtered load is returned to bloodstream, the amount in the urine relates directly to the volume of blood that filtered in the glomeruli. GFR can be calculated by dividing creatinine excretion rate (mg/hr) by plasma concentration of creatinine (g/dL) to get the GFR in dL/hour, from which the per-minute rate is calculated by dividing by 60. The test is not entirely accurate because up to 15 percent of the excreted creatinine enters the urine by tubular secretion.

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