Glucose Tolerance Specimen Collection

Instructions for Glucose Tolerance Specimen Collection

In order for glucose tolerance testing (GTT) to provide meaningful test data to the physician, test conditions must be rigidly controlled.

Patient Preparation:

The physician should avoid ordering the test on patients taking drugs that affect glucose metabolism. Interpretation of the glucose tolerance test can be confounded by certain medications such as salicylates, diuretics, nicotinic acid, β-adrenergic blocking agents, and high doses of certain hormones. When possible, medications known to affect glucose metabolism should be discontinued three days prior to the test. The GTT should be performed only on patients who have been on an unrestricted diet (including at least 150 g carbohydrate daily) and have had unrestricted 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 a minimum of 10 hours prior to the test and avoid even black coffee (water is permitted).

Collection of Samples:

Upon arrival, a fasting urine is collected for all patients except gestational women. The sample is sent to the laboratory to be screened for glucose using a dipstick. If positive, the fasting blood sample is collected and sent for stat analysis to the RRL associated with the Patient Service Center. If glucose is greater than 126 mg/dL, the test is cancelled and the ordering physician is notified.

If negative, the testing proceeds as follows:

• The patient is given an oral glucose dose and should drink the liquid within 5 minutes. The 0-hour for the test begins when the patient starts to drink the dose. During the test the patient should remain seated.
• Subsequent blood specimens should be collected in sodium fluoride/potassium oxalate (gray top) tube at 1-hour intervals for the duration of the tolerance requested.

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

1. Significant fasting hyperglycemia
2. Hospitalized, acutely ill, or inactive patients

References:

1. Diagnosis and Classification of Diabetes Mellitus. Diabetes Care 30, 2007.
2. Henry’s Clinical Diagnosis and Management by Laboratory Methods, 21st ed., 2007, pp 189-190.

Glucose Tolerance Test Specimen Collection:

Please call the Patient Service Center to schedule the testing in advance of the patient's visit.

For Pregnant Patients

ANY Laboratory Alliance Patient Service Center may be utilized.