A qualitative glucose test is part of routine urinalysis. This screening test for the presence of glucose within the urine may indicate the likelihood of diabetes mellitus or other causes of glucose intolerance (see Glucose, p. 969). This diagnosis must be confirmed by other tests (e.g., fasting glucose, glucose tolerance, glycosylated hemoglobin). Urine glucose tests may be used to monitor the effectiveness of diabetes therapy; however, today this is largely supplanted today by fingerstick determinations of blood glucose levels.
In patients with diabetes that is not well controlled with hypoglycemic agents, blood glucose levels can become very high. Normally, glucose is filtered from the blood by the glomeruli of the kidney. In the glomerular filtrate, the glucose concentration is the same as in the blood. Normally, all of the glucose is reabsorbed in the proximal renal tubules. When the blood glucose level exceeds the capability of the renal threshold to reabsorb the glucose (about 180 mg/dL), it begins to spill over into the urine (glycosuria). As the blood glucose level increases, the amount of glucose spilling into the urine also increases.
Glucosuria may occur immediately after eating a high-carbohydrate meal, and in patients with otherwise normal glucose levels or prediabetic patients receiving dextrose-containing intravenous (IV) fluids. Further, glucosuria does not always indicate diabetes but can occur normally or in diseases that affect the renal tubule or in genetic defects in metabolism and excretion of glucose. In these diseases, the renal threshold for glucose is abnormally low. Despite a normal blood glucose concentration, the kidney cannot reabsorb the normal glucose load. As a result, surplus glucose is spilled into the urine. In these patients, results of glucose tolerance tests are normal. Patients with acute severe physical stress or injury can have a transient glucosuria caused by normal compensatory endocrine-mediated responses.
In many cases, Urine Glucose test may be performed at specified times during the day, usually before meals and at bedtime. If patients hold their urine, doing so will not give an accurate indication of blood glucose levels at testing time. To assure a fresh urine specimen, a void urine sample can be taken and discarded 30 minutes before the test specimen is taken, the patient should drink a glass of water before the actual specimen it taken.
Casues of Glucose Urine False Results
Any substance that can reduce copper in the Clinitest can produce false-positive results. This may include other sugars glucose (e.g., galactose, fructose, lactose).
By using reagent tablets (Clinitest for example) the following drugs may cause false positive results:
Acetylsalicylic acid, aminosalicylic acid, ascorbic acid, cephalothin, chloral hydrate, nitrofurantoin, streptomycin, and sulfonamides. These drugs however don’t cause false positive results using enzyme-impregnated strips (Clinistix and Tes-Tape for example).
Urine Glucose levels can also be increased by taking the following drugs:
aminosalicylic acid, cephalosporins, chloral hydrate, chloramphenicol, dextrothyroxine, diazoxide, diuretics (loop and thiazide), estrogen, glucose infusions, isoniazid, levodopa, lithium, nafcillin, nalidixic acid, and nicotinic acid (large doses).
False negative results can be caused by taking drugs such like ascorbic acid (Using Clinistix and Tes-Tape), levodopa (when Clinistix is used), and phenazopyridine (with both Clinistix and Tes-Tape).
Normal Urine Glucose Test Results
Glucose shouldn’t be found in urine in a healthy person. In other words, the Urine Glucose test should show negative results. In cases where 24 hours specimen is required, the total glucose found in all specimens taken in a 24 hours period shouldn’t exceed 5 grams.
Causes of Urine Glucose Positive Results and High Levels
High Urine Glucose levels occur if the total glucose found in urine exceeds 5 grams when 24 hours testing is applied. The causes of positive and high Urine Glucose levels include:
Diabetes mellitus and other causes of hyperglycemia.
Pregnancy: Glycosuria is common in pregnant women. Persistent and significantly high levels may indicate gestational diabetes or other obstetric illness. Also, lactosuria is common in nursing women. Lactose is a reducing substance that may cause false-positive results for glucose, depending on the method of testing.
Renal glycosuria: It can occur normally or in patients with diseases that affect the renal tubule. It can also result from genetic defects in the metabolism and excretion of glucose. In these diseases, the renal threshold for glucose is abnormally low. Despite a normal blood glucose level, the kidney cannot reabsorb the glucose it should. As a result, the surplus glucose is spilled into the urine.
Fanconi syndrome: Associated with transport defects in the proximal renal tubules, causing glycosuria, this genetic defect can also affect the metabolism and excretion of amino acids and electrolytes.
Hereditary defects in metabolism of other reducing substances (e.g., galactose, fructose, pentose): These reducing substances may cause false-positive tests for glucose, depending on the method of testing.
Increased intracranial pressure (e.g., from tumors, hemorrhage): The pathophysiology for this observation is not well defined, although many theories exist.
Nephrotoxic chemicals (e.g., carbon monoxide, mercury, lead): These chemicals injure the kidney and lower the renal threshold.