Glucose Tolerance Test (GTT), Two-hour (Oral WHO Protocol)

Also known as: Fasting, Two-hour Glucose Tolerance Test, Oral Glucose Tolerance Test

Clinical Use

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

Not provided.

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.

Test Details

Methodology

Automated Analyzer (Clinical Chemistry)

Biomarkers

Glucose, 2 hour

Analyte

Glucose, 2 hour

Concentration / Level • Quantitative (Continuous) (e.g., copies/mL, % expression)

Glucose, Fasting

Analyte

Glucose, Fasting

Concentration / Level • Quantitative (Continuous) (e.g., copies/mL, % expression)

LOINC Codes

Order Codes

93794-6 - GTT ISI 2h Pnl SerPl

Result Codes

1558-6 - Glucose p fast SerPl-mCnc
1518-0 - Glucose 2h p 75 g Glc PO SerPl-mCnc

Result Turnaround Time

1 day

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)