Glycosuria refers to sugar in the urine. Less than 0.1% of glucose normally filtered by the glomeruli appears in the urine, and less than 130 mg should appear in the urine over a 24-hour period. Glucose is present in glomerular filtrate but is reabsorbed by the kidney's proximal tubule. If the blood glucose level exceeds the capacity of the tubules to reabsorb all the glucose present in the glomerular filtrate, the renal threshold is reached and glucose spills into the urine. A finding of glycosuria indicates that the person is hyperglycemic or has a lowered renal threshold for glucose. The renal threshold for glucose is approximately 160 to 190mg/dl of blood; glucose does not appear in the urine until the blood glucose rises above this level.

Occasionally, glycosuria may be a normal finding, such as after eating a heavy meal or during times of emotional stress. Some individuals have a benign condition in which they have a lower than usual renal threshold for glucose, but have normal blood glucose levels. In pregnancy, the renal threshold for glucose may be lowered so that small amounts of glycosuria may be present. Patients on hyperalimentation may have glycosuria if the carbohydrate solution is being infused faster than the pancreas can produce insulin. The most common reason for glycosuria is diabetes mellitus. Urine glucose tests are used to screen for diabetes, to confirm a diagnosis of diabetes, or to monitor diabetic control.