Instructions for Glucose Tolerance Testing

In order for glucose tolerance testing (GTT) to provide meaningful test data to the physician, test conditions must be rigidly controlled.  The physician should avoid ordering the test for patients taking drugs that effect glucose metabolism.  Interpretation of the glucose tolerance test can be confounded by certain medications such as salicylates, diuretics, nicotinic acid, b-adrenergic blocking agents, and high doses of some hormones.  When possible, medications known to effect glucose metabolism should be discontinued three days prior to the test.  The GTT should be performed only on patients who have been on unrestricted diet (including at least 150 g carbohydrate daily) and physical activity for three days before the test.  Two additional days of this diet are essential if the patient has been on a diet with insufficient carbohydrate intake.  The patient must fast for 10 to 16 hours prior to the test and avoid even black coffee (water is permitted).  The patient is given an oral glucose dose and should drink the liquid within 5 minutes of the collection of the fasting glucose sample.  The 0-hour for the test begins when the patient starts to drink the dose.  During the test the patient should remain seated and should refrain from smoking.  Blood specimens should be collected in sodium fluoride tubes (gray top).

 A glucose tolerance test should not be performed in the following circumstances:

1)     fasting hyperglycemia

2)     hospitalized, acutely ill, or inactive patients

References:

1.      Report of the Expert Committee on the Diagnosis and Classification of Diabetes Mellitus. 

         Diabetes Care       20, 1997.

2.      Henry. J.B. Clinical Diagnosis and Management, 19th ed., 1996, pp 200-201.