Measurement of glomerular filtration rate by low-dose iopromide plasma clearance

Glomerular filtration rate (GFR) can be measured by iopromide plasma clearance. As an injection of 120 ml nonionic contrast medium is expensive and especially in patients with nephropathy potentially nephrotoxic, we investigated whether iopromide plasma clearance could be measured using a tenth of that dose as 'low-dose' clearance.

Fifty adult patients scheduled for CT were recruited. Iopromide 300 mg I/ml was used for GFR measurement. Prior to CT, low-dose clearance was measured by injecting 12 ml iopromide per 75 kg b.w. At 3, 4 and 5 h after injection, plasma samples were obtained and the iodine concentration was measured by X-ray fluorescence analysis. Immediately after the last blood sample, CT was again performed following injection of 120 ml iopromide per 75 kg b.w. A further 3 plasma samples were then obtained 3, 4, and 5 h after CT and used for the determination of high-dose clearance.

Low-dose clearance ranged from 20 ml/min to 141 ml/min (mean 78.3 ml/min). High-dose and low-dose clearance correlated excellently, with clearance (high-dose) = 1.4 + 0.994 clearance (low-dose); the correlation coefficient was r = 0.944, the standard deviation SDxy= 9.3 ml/min.

Plasma clearance of iopromide is dose-independent on use of iodine amounts of 3.6 g and 36 g I/75 kg b.w. The GFR can be determined by high-dose and low-dose iopromide plasma clearance with identical accuracy.