GFR measurement with iohexol and 51Cr-EDTA.: A comparison of the two favoured GFR markers in Europe

Background. The aim of the present study was to compare the most commonly used GFR markers for clearance measurements, Cr-51-EDTA and iohexol, using two different methods for iohexol analysis, HPLC and X-ray fluorescence, referring both to the multiple-sample and single-sample calculations, using Cr-51-EDTA as the reference method. Methods. Forty-nine patients with an estimated GFR >40 ml/min were included. Cr-51-EDTA and iohexol were injected simultaneously and blood samples were taken 150, 195 and 240 min after injection of the respective marker. Results. The multiple-point clearances, determined from HPLC and X-ray fluorescence, compared to Cr-51-EDTA correlated highly (r=0.92 and 0.95 respectively). The results from single-point clearance comparison, iohexol measured by HPLC vs Cr-51-EDTA, yielded a correlation of r=0.91, while single-point clearance from iohexol, analysed by X-ray fluorescence, obtained a correlation of 0.93 and an intercept statistically different from origo. Conclusions. Iohexol and Cr-51-EDTA are comparable as GFR markers for multiple-point clearance measurements. The single-sample method for GFR >40 ml/min can be used with a high accuracy. The precision and accuracy of X-ray fluorescence analysis of low concentrations of iohexol were less than those of HPLC. Care should therefore be taken when using X-ray fluorescence that the injected dose of iohexol should result in a plasma concentration level of iodine of at least 0.06 mg/ml at the time of blood sampling.