Clinical Significance

Glucose measurements are used in the diagnosis and treatment of carbohydrate metabolism disorders including diabetes mellitus, neonatal hypoglycemia, idiopathic hypoglycemia, and pancreatic islet cell carcinoma. Oral glucose tolerance tests are performed to establish a diagnosis:

• In patients with transient or sustained glucosuria who have no clinical symptoms of diabetes (polyuria) and with normal fasting and postprandial blood glucose levels.
• In patients with symptoms of diabetes but with no glucosuria and normal fasting levels.
• In persons with a strong family history of diabetes, but with no overt symptoms.
• In patients whose glucosuria is associated with pregnancy, thyrotoxicosis, liver disease, and/or infections.
• In women who have characteristically large (>4000 gm) babies.
• In patients with neuropathies and retinopathies of undetermined origin.

Patients with mild or diet-controlled diabetes may have fasting blood glucose levels within the normal range, but be unable to produce sufficient insulin for prompt metabolism of ingested carbohydrate. As a result, blood glucose rises to abnormally high levels and the return to normal is delayed. In other words, the patient has a decreased tolerance for glucose. Therefore, glucose tolerance tests are most helpful in establishing a diagnosis of a mild case of diabetes.

When a standard dose of 100 grams of glucose is given orally, absorption occurs rapidly and the blood glucose concentration increases. In a normal patient, this stimulates the pancreas to produce more insulin with the result that after 30 to 60 minutes the blood level of glucose begins to decrease. Since there now exists more insulin than necessary, the blood glucose tends to drop below the fasting level after 1.5 to 2 hours, and then returns to normal levels around 3 hours. Response to glucose in various conditions is shown with a glucose tolerance curve.

In a normal response the fasting level of serum glucose is within normal limits, the peak concentration is reached 30 to 60 minutes and does not exceed 170 mg/dL. If the values in the first hour show a rapid fall to normal values or a flat curve with no appreciable rise usually reflect primary alterations in intestinal absorptive states. A very flat rise in blood glucose followed by a prolonged and pronounced hypoglycemic phase may be observed in primary and secondary hyperinsulinism.