Glomerular Filtration Rate (GFR) value is widely accepted as the best overall index of kidney function in health and disease. The aim of this standad operative procedure (SOP) is to determine the GFR by utilizing the plasma clearance of iohexol, a non-radioactive contrast agent. Briefly, on the morning of renal function evaluation, 5 ml of iohexol solution (Omnipaque 300, GE Healthcare, Milan, Italy) is injected intravenously over 2 minutes.
According to the expected GFR estimated by the CKD-Epi equation (eGFR), blood samples are taken at 120, 180, 240, 300, 360, 420, and 480 min for patients with eGFR ≤40 mL/min/1.73m2, and at 120, 150, 180, 210, and 240 min for those with eGFR >40 mL/min/1.73m2. Iohexol plasma levels are measured by high-performance-liquid chromatography (HPLC).
The clearance of the marker is first calculated according to a one-compartmentmodel (CL1) by the formula: CL1 = injected dose of iohexol/AUC where AUC is the area under the curve of the plasma concentration of iohexol. Then the value obtained is corrected according to Bröchner-Mortensen, in order to estimate the corrected GFR (plasma clearance).