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Background: The glomerular filtration rate (GFR) can be predicted from plasma creatinine, age, gender, and body weight, using the formula of Cockcroft and Gault. Cimetidine improved the accuracy of GFR prediction in renal disease and also in diabetes mellitus type 2, due to inhibition of tubular creatinine secretion. We compared the accuracy and precision of GFR prediction from the Cockcroft-Gault formula without cimetidine (CG), with cimetidine (CGcim) and from the creatinine clearance without cimetidine in renal transplant recipients., Methods: CG and CGcim were calculated from plasma creatinine before and after 2400 mg of oral cimetidine during the 24 hr preceding the GFR measurement. The endogenous creatinine clearance was measured in 24 outpatients from a 24-hr urine collection (Ccr24) before cimetidine. GFR was measured as the urinary clearance of continuously infused 125I-iothalamate. Creatinine was determined with an automated enzymatic assay in plasma and with an alkaline picrate assay in urine., Results: GFR was 47.8+/-16.8 ml/min/1.73 m2 (mean+/-SD), Ccr24 was 71.8+/-23.1 ml/min/1.73 m2, CG was 62.2+/-15.2 ml/min/1.73 m2, and CGcim was 52.8+/-14.9 ml/min/1.73 m2. Ccr24 overestimated GFR in every patient by an average of 23.8 ml/min/1.73 m2 and CG by an average of 14.3 ml/min/1.73 m2, whereas CGcim overestimated GFR significantly less by an average 4.9 ml/min/1.73 m2 (P<0.001). The precision of CGcim was significantly better than that of Ccr24: the SD of the difference from GFR was 9.0 ml/min/1.73 m2 for CGcim and 14.5 ml/min/1.73 m2 for Ccr24 (P<0.05)., Conclusion: CGcim is useful for GFR prediction in outpatient renal transplant recipients and has a far better accuracy and precision than Ccr24 and also a better accuracy than CG. We propose a strategy after kidney transplantation of one GFR measurement at baseline and follow-up with CGcim.