The invention provides a method involving assessing the vitamin C renal leak status of a human subject, vitamin C renal leak being the clinically inappropriate presence of vitamin C in urine. The method comprises assaying for the concentration of vitamin C in a plasma sample and in collected urine output, whereby the presence of vitamin C in the collected urine output and a concentration of vitamin C in the plasma sample less than the Minimum Elimination Threshold (MET) indicates the presence of vitamin C renal leak. Vitamin C renal leak also can be quantified by determining the fractional excretion of vitamin C (FeVC), which is a measurement of the degree of urinary vitamin C excretion relative to plasma vitamin C concentration corrected by creatinine clearance. The invention also provides a test kit for assessing vitamin C renal leak and a test kit for assessing FeVC.