Multicenter Laboratory Comparison of Iohexol Measurement

Area of Science:

• Clinical Chemistry
• Nephrology
• Laboratory Medicine

Background:

• Iohexol is a contrast agent used for measuring kidney glomerular filtration rate (GFR).
• Lack of standardized proficiency materials previously hindered accurate laboratory calibration for iohexol assays.
• A calibration shift at the University of Rochester Medical Center (URMC) prompted a multi-center laboratory comparison.

Purpose of the Study:

• To compare iohexol measurements across multiple laboratories.
• To assess the impact of calibration differences on GFR results.
• To evaluate the utility of proficiency testing materials for inter-laboratory agreement.

Main Methods:

• Duplicate analysis of fortified sera and patient samples were performed at URMC, University of Minnesota (UMN), Mayo Clinic, and University of Lund.
• Proficiency testing materials from Equalis AB were utilized.
• Iohexol calibration involved dilutions of Omnipaque™ 300, with concentrations measured by HPLC or LC-MS/MS.

Main Results:

• Initial comparisons showed URMC and Mayo calibrations were lower than UMN and Lund.
• After correction, URMC calibration became higher for fortified samples.
• Adjusting measurements using Equalis proficiency materials resulted in agreement within 1-2% across all laboratories.

Conclusions:

• URMC calibrator lots from 2006-2016 underestimated iohexol concentration by a factor of 0.89, potentially reducing calculated GFRs by 10-11%.
• GFR results for the CKiD study were adjusted to account for this calibration shift.
• Regular proficiency testing, sample exchange, and standardized calibration are recommended for universal assay agreement.