Introduction Renal clearance describes the amount (volume) of plasma that is completely cleared by the kidneys per unit time in other words it reflects the excretion of a substance (e.g., medications) this depends on glomerular filtration tubular reabsorption and secretion clearance decreases gradually with age but may have acute changes due to pathology equation C(x) = (UxV) / Px C(x) = clearance of substance X (mL/min) Ux = urine concentration of substance X (mg/mL) V = urine flow rate (mL/min) Px = plasma concentration of substance X (mg/mL) Clearance ratios C(inulin) C(inulin) = GFR inulin is freely filtered across glomerular capillaries it is neither reabsorbed or secreted amount of inulin filtered equals amount of inulin excreted inulin is a reference substance called a glomerular marker C(x) / C(inulin) > 1.0 C(x) > GFR interpretation there is a net secretion of substance X e.g., para-aminohippuric acid can be used to determine renal plasma flow C(x) / C(inulin) < 1.0 C(x) < GFR interpretation there is a net reabsorption of subtance X e.g., glucose