Iohexol plasma clearance simplified by dried blood spot testing

Background. Renal function can be estimated with formulas, which are inaccurate, or measured with gold standard methods, which are reliable but unpractical. We propose to simplify the plasma clearance of iohexol, a gold standard method to measure renal function, by dried blood spot (DBS) testing. Methods. We compared glomerular filtration rate (GFR) values assessed by DBS and the reference plasma analysis technique. We tested in vitro the agreement between non-volumetric and volumetric DBS with the reference technique. Then, we performed a clinical validation in vivo between volumetric DBS and plasma analysis in 203 patients. The agreement was evaluated with the concordance correlation coefficient (CCC), the total deviation index (TDI) and the coverage probability. We defined acceptable agreement as a TDI <10%. Results. In the in vitro studies, the non-volumetric DBS showed moderate agreement, TDI = 26.0%, while the volumetric method showed better but insufficient agreement, TDI = 13.0%, with the reference method in plasma. The non-volumetric DBS was rejected. To improve the agreement of the volumetric DBS, iopamidol was used as an internal standard. This method showed acceptable agreement, TDI = 9.0% with the analysis in plasma, and was selected as the definitive DBS method. In the in vivo studies, the agreement between the final DBS method and the reference technique was acceptable: TDI = 9.5%. This indicates that 90% of the GFR values ranged from -95% to + 9.5% compared with the reference method. Conclusions. We simplified the plasma clearance of iohexol using DBS without losing accuracy and precision with respect to the reference technique. This may facilitate the use of a reliable determination of renal function to the medical community.