The nonionic low-osmolar contrast medium iohexol was used as marker of glomerular filtration rate (GFR) in 53 patients with stable renal function (group I: n = 32, group II: n = 21). All the patients had clearance values < or = 30 ml.min-1.1.73 m-2 body surface; 40 patients < 20 ml.min-1.1.73 m-2 body surface. Simultaneous determinations of renal clearance and plasma clearance, both as slope clearance and single sample clearance, were performed after intravenous injection of 10 ml iohexol 300 mg iodine/ml. In groups I and II plasma was sampled early (around 3 hours) and late (up to 24 hours) after the injection. In group I urine was collected during four 40-minute periods and in group II during one 3-hour period and in group II the residual urine was estimated by ultrasound. Plasma and urine iodine concentrations were analyzed with X-ray fluorescence technique (Reanalyzer PRX90, Provalid AB, Sweden). In group II S-creatinine and tubular function test were followed to detect any signs of nephrotoxicity. In 6 anuric patients (group III) 10 ml iohexol 300 mg I/ml was injected to assess its extrarenal clearance. In groups I and II the slope clearance correlated excellently with the single sample clearance (r = 0.99) when a late plasma sample was used in both techniques. In group II, where residual urine was estimated by ultrasound, renal clearance correlated better with slope clearance than in group I (r = 0.94 vs r = 0.89). There were no signs of nephrotoxicity in the parameters noted. In group III, extrarenal plasma clearance of iohexol did not exceed 2 ml.min-1.1.73 m-2.
Conclusion: GFR < 20 ml/min can accurately and safely be determined as renal clearance or plasma clearance of iohexol after an intravenous dose of 10 ml 300 mg I/ml. Plasma clearance techniques, which have the practical clinical advantage of no urine sampling, do at low GFR require a late plasma sample taken, for instance, 24 hours after injection of iohexol, irrespective of whether slope technique or single sample technique or one-compartment or poly-compartment models are used.