Cimetidine inhibits the tubular secretion of creatinine, without altering the glomerular filtration rate (GFR). During cimetidine administration the creatinine/inulin clearance ratio approaches unity in patients with renal failure. We determined the clearance of lithium (an index of fluid delivery to the distal nephron), inulin (a measure of the actual GFR) and creatinine during cimetidine administration to investigate the occurrence of tubular creatinine secretion in patients with compensated cirrhosis. A total of 12 patients with Child-Pugh A cirrhosis were studied initially. The subjects consumed a stable diet containing 100mmol of sodium. On successive days, 9h creatinine clearances were measured, first without and then with the oral administration of cimetidine (400mg as a priming dose, followed by 200mg every 3h). During the first study day, 4h renal lithium clearance was also calculated. A further group of five patients with fully compensated cirrhosis underwent the measurement (on successive days) of plasma inulin clearance, first without and then with the oral administration of cimetidine (same schedule of drug administration). Cimetidine administration unmasked a marked overestimation of GFR when calculated as creatinine clearance (baseline, 138±20ml/min; +cimetidine, 89±13ml/min; P < 0.01). Consequently, during cimetidine administration the calculated lithium fractional excretion (a measure of the fraction of filtered sodium load that is delivered to the loop of Henle) rose from 21.4±13.2% to 32.3±18.9% (P < 0.05), and the ratio between absolute distal tubular sodium reabsorption and filtered sodium load rose from 20.6±13.1% to 31.6±19.3% (P < 0.01). Cimetidine caused no significant decrease in the actual GFR (i.e. inulin clearance) when administered to the second group of patients with compensated cirrhosis. Our data demonstrate significant tubular secretion of creatinine in patients with compensated cirrhosis and, consequently, a marked overestimation of GFR and filtered sodium load and an underestimation of the fractional distal tubular sodium reabsorption when these parameters are calculated by means of the traditional creatinine and lithium clearance computation. The true GFR (measured as inulin clearance) is unaffected by cimetidine administration.

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