Glucose Tolerance Test (GTT), Two-hour (Oral WHO Protocol)
Also known as: Fasting, Two-hour Glucose Tolerance Test, Oral Glucose Tolerance Test
Use
The GTT only establishes the presence of glucose intolerance. It is used in patients with borderline fasting and postprandial glucose to support or rule out the diagnosis of diabetes mellitus. Some use it in unexplained hypertriglyceridemia, neuropathy, impotence, diabetes-like renal diseases, retinopathy, reevaluation of prior diagnosis made under substandard conditions and with necrobiosis lipoidica diabeticorum.
Special Instructions
For accurate results, patient preparation is vital: adhere to a minimum three-day high carbohydrate diet and ensure fasting before the test. Decision to cancel due to emesis lies with the patient's physician. Certain medications affecting glucose metabolism should ideally be paused three days before testing. Proper specimen labeling at collection time intervals is crucial to avoid rejection.
Limitations
The GTT primarily identifies glucose intolerance, not diabetes mellitus. It shows slight hyperglycemic effects in those on oral contraceptives, and the absence of a high carbohydrate diet could lead to false positives. Results lack specificity for diabetes complications and indicate increased cardiovascular disease risk in those with impaired glucose tolerance. It does not predict future diabetes development, and presence of obvious diabetes mellitus contraindicates testing.
Methodology
Automated Analyzer (Clinical Chemistry)
Biomarkers
LOINC Codes
- 93794-6
- 1558-6
- 1518-0
Result Turnaround Time
1 day
Related Documents
For more information, please review the documents below
Specimen
Serum
Volume
1 mL
Minimum Volume
0.5 mL
Container
Gel-barrier tubes (2) or gray-top (sodium fluoride/potassium oxalate plasma) tubes (2)
Collection Instructions
Draw a fasting blood sample before administering glucose. Administer a 75-gram glucose and draw blood after two hours. The patient should remain seated throughout the test.
Patient Preparation
Patient should be active and eat a regular diet with at least 150 grams of carbohydrate daily for three days prior to the test. Fast for at least eight hours, not more than 14 hours before test. Discontinue nonessential medication affecting glucose metabolism if possible three days before testing.
Storage Instructions
Maintain specimen at room temperature.
Causes for Rejection
Stressed patient (surgery, infection, corticosteroids) should not have GTT; specimens not labeled with collection time intervals (e.g., fasting, and two-hour)
Stability Requirements
| Temperature | Period |
|---|---|
| Room Temperature | 14 days |
| Refrigerated | 14 days |
| Frozen | 14 days |