#4179 DIFFERENCES IN PROTOCOLS FOR MEASURING GLOMERULAR FILTRATION RATE USING IOHEXOL

Abstract

Background and Aims
When assessing kidney function, measurement of glomerular filtration rate (mGFR) using an exogenous marker such as iohexol is the gold standard. In this study, we aim to identify similarities and differences between iohexol-based mGFR protocols.

Method
Detailed data on iohexol measurement protocols were obtained using a standardized survey sent to centres in Europe and the US. It was completed by 15 participants. Data are reported as number and percentage (n, %).

Results
In the participating centres, measurements are performed after referral by a nephrologist (n=6, 40%), other specialties (n=5, 33%) or for research purposes (n=4, 27%). Most common indications for mGFR are evaluation of kidney donors (n=5, 33%), drug dosing (n=4, 27%), abnormal body composition (n=3, 20%) and transplant evaluation (n=4, 27%). Most participants perform measurements in the morning (n=10, 67%), with patients withholding caffeine (n=5, 33%), fasting (n=4, 27%) or avoiding heavy meals (n=3, 20%). Most centres use an IV iohexol dose of 5 mL 300 mg I/mL (n=10, 67 %) or a dose based on weight (n=2, 13%). The timing of sample collection is shown in Figure 1, most often a single sample per time point (n=12, 80%). Iohexol is measured by LC-MS (n=8, 53%) or LC-UV (n=7, 47%). Within-assay variability ranges between <2% (n=3, 20%) and 6-8% (n=1, 7%) and the between-assay variability ranges between <2% (n=2, 13%) and 6-8% (n=2, 13%). When asked about assumptions and corrections, most centres make a one-compartment assumption in their PK model (n=8, 53%), others making two-compartment (n=2, 13%) or measurement-dependent (n=1, 7%) assumptions. mGFR is standardized for body surface area according to the DuBois-formula (n=6 (40%), Haycock and Schwarz formula (n=2, 13%) or not-specified (n=7, 47%). Some participants correct their measurements for eGFR (n=9, 60 %). Most centres participate in external quality control (Equalis, n=12, 80%).

Conclusion
There is a large variation in protocols for iohexol-based mGFR, which highlights the need for a standardized mGFR protocol before widespread use in clinical routine.