Glucose is not present in urine, in general because in the kidneys, glucose is reabsorbed from the filtrate of glomerulus, across the tubular epithelium of proximal tubule into the bloodstream. Therefore, normal glucose range in urine is 0 mg/dL, or rarely up to 15 mg/dL. If higher than normal levels of glucose (0-15 mg/dL) is found in urine, then it can be suspected as diabetes, pregnancy, or renal glycosuria. Presence of glucose in urine was commonly tested to monitor diabetes in the past. Now, blood tests for glucose is easier than urine test. The glucose urine test may be done when there is suspect of renal glycosuria.
One of the test is based on the basis of character of glucose as it is a reducing carbohydrate because of a free aldehyde group. During the reduction process of Cu(OH)2 to CuOH by the glucose aldehyde group, and itself is converted into a carboxyl group. CuOH and its degradation product, Cu2O, are colored compounds. In excess of CuSO4, the high Cu(OH)2 content may lead to the formation of black CuO particles. This is called Trommer’s reaction and explained in the following equations;