Endogenous creatinine clearance as a clinical measure of glomerular filtration rate.

Endogenous creatinine clearance was assessed by inulin in 91 simultaneous studies in 41 patients and proved an accurate measurement of glomerular filtration rate. For filtration rates <20 ml/min the creatinine clearance may slightly overestimate the glomerular filtration rate. Successive determinations of filtration rate by either method may vary by 20% even though the methods agree with each other. Glomerular filtration rate itself is variable, and in its clinical usage this should be recognized.