Iohexol plasma clearance for measuring glomerular filtration rate in clinical practice and research: a review. Part 1: How to measure glomerular filtration rate with iohexol?

While there is general agreement on the necessity to measure glomerular filtration rate (GFR) in many clinical situations, there is less agreement on the best method to achieve this purpose. As the gold standard method for GFR determination, urinary (or renal) clearance of inulin, fades into the background due to inconvenience and high cost, a diversity of filtration markers and protocols compete to replace it. In this review, we suggest that iohexol, a non-ionic contrast agent, is most suited to replace inulin as the marker of choice for GFR determination. Iohexol comes very close to fulfilling all requirements for an ideal GFR marker in terms of low extra-renal excretion, low protein binding and in being neither secreted nor reabsorbed by the kidney. In addition, iohexol is virtually non-toxic and carries a low cost. As iohexol is stable in plasma, administration and sample analysis can be separated in both space and time, allowing access to GFR determination across different settings. An external proficiency programme operated by Equalis AB, Sweden, exists for iohexol, facilitating interlaboratory comparison of results. Plasma clearance measurement is the protocol of choice as it combines a reliable GFR determination with convenience for the patient. Single-sample protocols dominate, but multiple-sample protocols may be more accurate in specific situations. In low GFRs one or more late samples should be included to improve accuracy. In patients with large oedema or ascites, urinary clearance protocols should be employed. In conclusion, plasma clearance of iohexol may well be the best candidate for a common GFR determination method.