Glomerular filtration rate is a true index for measuring renal function. More commonly, however, glomerular filtration rate is estimated using creatinine clearance. Glomerular filtration rate in adults can be estimated from the patient's serum creatinine, weight, age, and gender by using the Cockroft and Gault equation ( Cockroft and Gault... .197,6):
where Ccr is the creatinine clearance (ml/min), IBW is the ideal body weight (kg), and Scr is the serum creatinine (mg/dl = (^mol/l)/88.4). The ideal body weight is defined as follows:
IBW(men) = 50 kg + 2.3 kg per inch over 5 feet (0.9 kg per cm over 152 cm) IBW(women) = 45.5 kg + 2.3 kg per inch over 5 feet (0.9 kg per cm over 152 cm)
Serum creatinine is produced at a constant rate and has relevant characteristics that allow it to be used in the estimation of glomerular filtration rate. This equation should be used only when serum creatinine is stable and there are not continuing changes in renal function. The Cockroft and Gault equation is subject to significant error in patients with diminished muscle mass (the elderly, those with hepatic cirrhosis, and malnourished patients) (St. Peter 1993). In unstable patients, the estimated glomerular filtration rate is often an educated guess based on individual patient characteristics.
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