Multicenter Laboratory Comparison of Iohexol Measurement

Background: Iohexol is used for measurement of kidney glomerular filtration rate (GFR). Until recently, there have not been available proficiency standards to assist in calibrating a laboratory's results. In view of a shift in calibration at the University of Rochester Medical Center (URMC) laboratory, serving as Central Biochemistry Laboratory for the CKiD study, we performed a multicentered laboratory comparison. Methods: Two batches of 30 fortified sera and patient samples from serum or heparinized plasma were sent for duplicate analysis to URMC, University of Minnesota (UMN), Mayo Clinic, and University of Lund. Five proficiency testing materials from Equalis AB were also provided. Iohexol calibration was performed using dilutions of OmnipaqueTM 300 and concentrations measured by HPLC or LC-MS/MS (Mayo). Results: The 2 batches sent to UMN and University of Lund agreed well. URMC calibration was 11%-13% lower, and Mayo was 4%-8% lower for fortified samples. URMC corrected calibration was 3%-8% higher for these samples. When measured values were adjusted for the results of the Equalis samples, all laboratories agreed within 1%-2% on all iohexol concentrations. Conclusions: For 12 URMC calibrator lots from November 2006 to March 2016, the factor quantifying the underestimation of measured to true iohexol concentration was 0.89. If each concentration was divided by 0.89, the calculated GFRs would be reduced by 10%-11%. GFR results for CKiD were adjusted for this shift in calibration. Regular examination of iohexol proficiency testing materials, free exchange of samples among laboratories, and standardized dilution of the stock iohexol for calibration would help to bring more universal agreement to this assay.