Validity of rapid estimation of glomerular filtration rate in type 2 diabetic patients with normal renal function

Background. Rapid estimation of the renal function is widely used in clinical practice. Methods. The validity of rapid estimation of renal function as well as long-term changes in renal function from the Cockcroft-Gault formula for estimation of creatinine clearance or from serum creatinine measurements was evaluated against the Cr-51-EDTA plasma clearance technique in 36 type 2 diabetic patients with normal renal function followed for 5.2 (2.7-7.5) (mean range) years. Results. Compared with Cr-51-EDTA plasma clearance the Cockcroft-Gault formula significantly underestimated glomerular filtration rate by 21%. The degree of underestimation was observed over the whole range of glomerular filtration rate studied and increased with increasing levels of isotopically measured glomerular filtration rates. Serum creatinine was not significantly associated with glomerular filtration rate. The average long-term change in renal function was significantly overestimated by the Cockcroft-Gault formula (-2.8+/-2.3 mi/min/year) compared with the measured rate (- 1.5 +/-2.5 mi/min/year) (P=0.002). The difference in change rate between the two methods was highest when the measured fall rates were small and tended to disappear in patients with faster fall rates. Changes in serum creatinine correlated significantly, but imprecisely, with the rate of decline of measured glomerular filtration rate (r= -0.48, P=0.003). The variability of the estimated fall rate of renal function was unacceptably high for all approaches. Conclusion. Valid estimates of glomerular filtration rate as well as the rate of change in glomerular filtration rate cannot be obtained by estimation of creatinine clearance from the Cockcroft-Gault formula or from serum creatinine concentration measurements in type 2 diabetic patients with normal renal function.