Contrast media and glomerular filtration: dose dependence of clearance for three agents

Determination of plasma clearance of contrast agents has been advocated as a means to assess glomerular filtration rate. To evaluate the feasibility of different agents for this purpose, we have compared, in healthy volunteers, the dose dependence of plasma clearance for three contrast media (iohexol, a nonionic agent, and iothalamate and metrizoate, which are ionic substances), with special emphasis on the lower dose range (2-20 mL corresponding to 0.9-12.9 g, depending on dose and agent). Iohexol and iothalamate were cleared at constant rates, irrespective of given dose, whereas metrizoate clearance increased significantly at lower doses. In general, the clearances or iothalamate and metrizoate were, respectively, moderately and markedly higher than that of iohexol. The clearance of different doses of metrizoate (2 mL versus a radiographic dose of 40 mL or more) was also compared with the clearance of [51Cr]EDTA in two groups of patients with reduced renal function. When compared with [51Cr]EDTA in patients with renal dysfunction, metrizoate was cleared significantly faster after a 2-mL dose, whereas clearances were identical when the metrizoate dose was 40 mL or more. These findings indicate that tubular secretion plays an active role in the elimination of metrizoate. The pharmacokinetic properties of iohexol, in combination with its low toxicity, make it a suitable agent for determination of glomerular filtration rate in clinical practice.