Glucose Tolerance Test (GTT), 2 Specimens

The glucose tolerance test is a medical test in which glucose is given and blood samples taken afterward to determine how quickly it is cleared from the blood. The test is usually used to test for diabetes, insulin resistance, impaired beta cell function, and sometimes reactive hypoglycemia and acromegaly, or rarer disorders of carbohydrate metabolism. In the most commonly performed version of the test, an oral glucose tolerance test (OGTT), a standard dose of glucose is ingested by mouth and blood levels are checked two hours later.

Patient should be active and eat a regular diet that includes at least 150 grams of carbohydrate daily for three days prior to the test. Patient should be instructed not to eat or drink anything except water for at least eight hours and not more than 14 hours before the test. Patients should also be advised to discontinue, whenever possible, all nonessential medication that can affect glucose metabolism at least three days before testing.

3 DAYS BEFORE THE OGTT: Plan on eating three healthy meals and snacks for 3 days before the test. You do not need to buy special food, but you do need to make sure you have healthy foods to eat. Your meals should be balanced with plenty of carbohydrates.

Foods containing carbohydrates include:

Fruits

Breads

Cereal

Pasta

Rice

Crackers

Starchy vegetables (corn/peas/carrots)

12 HOURS BEFORE THE OGTT: DO NOT eat, smoke, or do heavy exercise 12 hours before the test. (For example: If your test is scheduled for first thing in the morning, ie: at 8 AM—Do not eat, smoke, or do heavy exercise after 8pm the night before). You may drink plain, NOT flavored, water.

AT THE LAB:

First: A fasting blood glucose test is done. This is a simple blood test that checks your blood sugar before you drink the glucose drink.

Next: The lab technician will tell you to drink the glucose drink. It will taste very sweet. It’s important to drink the whole amount fairly quickly.

For appropriate interpretation of this test, the patient must fast overnight and ingest a 75 g load of glucose, immediately after, a fasting specimen is obtained. The lab technician will tell you to drink the glucose drink. It will taste very sweet. It’s important to drink the whole amount fairly quickly.

Waiting: After you finish drinking all of the glucose drink, you’ll be asked to sit until it’s time for your next blood test (about 2 hours later). You may read, listen to music, talk, or do another quiet activity while you are waiting.

Fasting plasma glucose (measured before the OGTT begins) should be below 6.1 mmol/L (110 mg/dL). Fasting levels between 6.1 and 7.0 mmol/L (110 and 125 mg/dL) are borderline ("impaired fasting glycaemia"), and fasting levels repeatedly at or above 7.0 mmol/L (>126 mg/dL) are diagnostic of diabetes.

For a 2 hour GTT with 75 g intake, a glucose level below 7.8 mmol/L (140 mg/dL) is normal, whereas higher levels indicate hyperglycemia. Blood plasma glucose between 7.8 mmol/L (140 mg/dL) and 11.1 mmol/L (200 mg/dL) indicate "impaired glucose tolerance", and levels above 11.1 mmol/L at 2 hours confirm a diagnosis of diabetes.