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What Is Creatinine?
Creatinine is a substance that is produced in the breakdown of a chemical called creatinine phosphate. This reaction occurs in the muscle on a daily basis, and almost 1-3% of creatinine is broken down by the muscle. Since it is a waste product, the kidneys will filter out a large percentage of the creatinine from the blood into the urine. Therefore, the amount of creatinine in the urine is a useful indicator for determining how well the kidneys are functioning.
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Determining The Creatinine Clearance
The rate at which the kidneys remove creatinine from the blood is called the creatinine clearance, and this value is directly correlated to the kidney’s glomerular filtration rate. This value is measured over a twenty-four period of collecting urine, and if the amount of creatinine in the urine is decreased, this may indicate abnormal kidney function.
For laboratory testing of creatinine, the urine is collected in the morning after they go to the bathroom to empty their bladder. This time is recorded, and from then on urine collection will start, with all subsequent urine being placed in the container that was given for urine collection. The time is recorded for the last urine sample added to the container, and the container can be refrigerated.
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What Is The Estimated Creatinine Clearance?
Instead of actually measuring the creatinine clearance over a twenty-four hour period, it is possible to use the Cockcroft Gault formula to determine the estimated creatinine clearance. It is important to have measured the person's blood serum creatinine level, and the formula subtracts the person’s age from 140 and multiplies it by the weight in kilograms and a constant to determine the estimate. Therefore written below is the Cockcroft Gault Formula:
ECcr = (140-Age) X Mass (kg) X Constant
Serum Creatinine (in umol/L)
The constant for men and women is 1.23 and 1.03 respectively, and is designed to calculated the estimated creatinine clearance in adults.
It is believed that using the Cockcroft Gault formula will produce a more accurate value of a person’s creatinine clearance, whereas the measured values on a twenty-four period can be highly variable depending on how the factors at work during urine collection.
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Print Source: Davidson, Stanley & C. Haslett. 2002. "Davidson’s Principles and Practice of Medicine.” Churchill Livingstone, Edinburgh.
Web Source: Karger Medical and Scientific Publishers. "Estimated Creatinine Clearance By The Formula of Gault and Cockcroft in Renal Transplantation.” 1989. Available: http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowPDF&ArtikelNr=185340&Ausgabe=241539&ProduktNr=223854&filename=185340.pdf