GLOMERULAR-FILTRATION RATE DETERMINED FROM A SINGLE PLASMA SAMPLE AFTER INTRAVENOUS IOHEXOL INJECTION - IS IT RELIABLE

The iohexol injection plasma clearance method is a good alternative to the inulin clearance method for determination of GFR, but requires mu ltiple blood samples, To avoid this, methods have been developed which derive GFR from a formula that uses a single plasma concentration of the tracer and anthropometric data. The aim of this study was to evalu ate whether a single plasma sample taken after iohexol injection allow s reliable estimation of GFR, In this study, results of single-point d etermination were compared with those obtained by multiple-point plasm a clearance. The GFR of 686 outpatients with different degrees of rena l function were recalculated by use of the Jacobsson formula, The opti mum time for sampling was found at 10 h after injection of the marker for clearances <40 mL/min per 1.73 m(2), 4 h for clearances between 40 and 99 mL/min per 1.73 m(2), and 3 h for clearances >100 mL/min per 1 .73 m(2). Results documented that for 75% of the patients, the simplif ied technique gave an error between -5% to +5% in the evaluation of GF R; for the remaining 25% of the patients, prediction error ranged from -22% to +40%, Furthermore, despite a highly significant correlation b etween multiple-point iohexol clearance (six plasma samples) and the s ingle-point method (Y = 0.968X + 1.704, r(2) = 0.988), the regression intercept was statistically different from 0 and the standard error of the slope estimate established that 95% confidence interval did not i nclude 1.0 (the line of identity), thus indicating that the model can be rejected by the data at a significance level of 0.05. Thus the sing le-plasma-sample method to determine GFR after radio-contrast injectio n does not represent a real advantage over the multiple-point method a nd may lead to unacceptable errors in GFR calculation.