Iohexol is regarded as an accurate filtration marker for measuring GFR. Recently, Soveri et al1 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 al2 recently highlighted discordance between urinary clearance of iothalamate and iohexol measured by LC-MS/MS; iohexol clearance was ∼15% lower than iothalamate clearance.