We agree with Drs Sterner and Prütz1 that delaying collection of a late plasma sample may allow better estimation of plasma clearance of iohexol (CLiohexol) in patients with low glomerular filtration rates (GFRs). However, a late sample is not always feasible, for example, in patients treated with peritoneal dialysis (7 days per week) or frequent hemodialysis (4-7 days per week). Furthermore, relatively little is known about extrarenal CLiohexol in humans, including its source (presumably hepatic), variability, and dependence on serum iohexol concentrations.