 Glomerular filtration rate GFR is widely accepted as the best index of kidney function in health and disease, and accurate values are required for optimal clinical decision making and in large-scale epidemiologic studies and clinical trials. Current recommended methods for measuring GFR are expensive, cumbersome to administer and assay and requires urine collection and administration of radioactive materials. The purpose of this study is to evaluate two non-radioactive markers iohexol and gadolinium compared to urinary clearance of inulin and iothalamate. The investigators hypothesize that plasma clearance of non-radioactive markers will be unbiased compared to a gold standard and more precise than currently used urinary clearance methods. 