Substantial evidence demonstrates sCr, alone or used to estimate glomerular filtration rate (eGFR), overestimates kidney filtering function when compared with direct measurement of GFR [3]. A new, commonly used equation for estimating the GFR uses values of sCr, blood urea nitrogen (BUN), and cystatin C. Conversely, a direct measurement of kidney filtering function (mGFR) can be accomplished by determining the plasma (part of the blood) iohexol (Omnipaque300, GE Healthcare) clearance rate (rate at which the iohexol is removed from the body by the kidneys) over a period of 300 minutes, and has been documented as a sensitive and accurate measure of renal filtering function [4]. Plasma clearance of iohexol is an optimal method of determining mGFR, particularly in children, because it is not radioactive, is non-ionic, has a low osmolality, has been proven to be a safe and non-toxic X-ray contrast medium, and is excreted from the plasma exclusively by the kidneys [5]. Iohexol is a well-known contrast agent that is already FDA-approved for use in urographic procedures, angiographic procedures, and for the measurement of GFR (See Appendix I for more information on the history and safety of iohexol use for GFR measurement). Iohexol clearance from plasma has been shown in several studies to be a suitable method of determining GFR in adults [6] and children [7, 8, 9].