CIMETIDINE IMPROVES GFR-ESTIMATION BY THE COCKCROFT AND GAULT FORMULA

In some patients with renal disease 24-hour cimetidine aided creatinin e clearances cannot equal GFR even after administration of the maximum daily dose of cimetidine. Short duration cimetidine aided creatinine clearances can equal GFR but are inconvenient for clinical use and can be inaccurate due to incomplete urine collection. We studied how accu rately GFR can be estimated without the need to collect urine, by appl ying the Cockcroft and Gault formula (C-Cock) on a single plasma creat inine concentration, after oral administration of 3 X 800 mg cimetidin e during the preceding 24 hours. GFR was measured as standard clearanc e, using continuous infusion of I-125-iothalamate. Nineteen patients w ith various renal diseases, plasma creatinine <180 mu mol/l and body m ass index between 15 and 30 kg/m(2) were included. After cimetidine ad ministration, plasma creatinine values remained stable for 6 hours, de spite rapidly decreasing plasma cimetidine values during the same peri od, in all 15 patients with GFR >40 ml/min/1.73 m(2). Tubular creatini ne secretion was blocked completely in 14 of them, With cimetidine bot h accuracy and precision of the Cockcroft clearance improved: the mean (+/-SD) ratio of C-Cock to GFR decreased from 1.28 (+/-0.21) to 0.98 (+/-0.11) (p <0.001) and the standard deviation of the difference (C-C ock - GFR) decreased from 9.23 to 7.07 ml/min/1.73 m(2) (p <0.05). Wit h cimetidine the Cockcroft clearance con-elated well with GFR (r = 0.9 74, p <0.001) and this was as good as the correlation between GFR and a 4-hour standard creatinine clearance (r = 0.972, p <0.001). in concl usion, with a minimum of inconvenience, this method provides the clini cian with accurate information on GFR for the outpatient follow-up of patients with a mild-to-moderate decrease in renal function, provided that no gross discrepancy between total bodyweight and muscle mass is present.