Concordance Between Iothalamate and Iohexol Plasma Clearance.

To the Editor: Iohexol is regarded as an accurate filtration marker for measuring GFR. Recently, Soveri et al concluded that plasma clearance of iohexol could be considered a valid alternative to the gold standard, urinary clearance of inulin. Because inulin urinary clearance is cumbersome and costly, iohexol and iothalamate clearance have become popular in Europe and the United States, respectively. To advance GFR research, it therefore seems relevant to directly compare these 2 methods. Seegmiller et al recently highlighted discordance between urinary clearance of iothalamate and iohexol measured by LC-MS/MS; iohexol clearance wasw15% lower than iothalamate clearance. However, at least for iohexol, plasma disappearance is generally used in clinical practice and research. Here, we concomitantly measured GFR by plasma clearance of both iohexol and iothalamate in 102 patients. Indications for GFR measurement were kidney function evaluation in either potential living kidney donors or patients whose serum creatinine–based GFR estimation was suspected as inaccurate. The protocol had ethics committee approval (Belgian number: B7072014220701). All patients signed informed consent. Five mL of iohexol (Omnipaque 240, 240 mg/mL; GE Healthcare BVBA) and 5 mL of iothalamate (Conray 30; iothalamate meglumine, 141 mg/mL; Covidien) were administered simultaneously in the same vein (antecubital, forearm, or hand vein) and flushed with 10 mL of normal saline solution. Blood samples were obtained using the contralateral arm at 120, 180, 240, and 300 minutes after injection and GFR was calculated as per Brochner-Mortensen. Samples were centrifuged for 10 min at 3,000 g within 2 hours of collection and stored at280 C. Iohexol was measured by 2 methods (HPLC, as previously described, and LC-MS/MS); iothalamate was measured by LC-MS/MS (analytical methods in Item S1). Our laboratory participates in interlaboratory quality control performed by Equalis AB (Uppsala, Sweden) using both methods 4 times a year. Clearance results were analyzed by Passing-Bablok regressions and concordance correlation coefficient (CCC). For Bland-Altman analysis, we calculated relative bias as the mean