Glucose Tolerance (GTT) Blood Test, 2-Hour (Oral WHO Protocol)
The test requires individuals to fast for 10 – 12 hours before giving blood for the test. Individuals are allowed to drink water and to take medications during the fast.
Why It Is Used
The glucose tolerance, 2hr test is principally used to test for and diagnose Types 1 and 2 diabetes. The test can also be used to track the progression of treatment.
How It Works
This test involves two blood draws and is conducted over the course of two hours. The blood test requires 10 – 12 hours of fasting. The first blood draw will be taken immediately upon arrival to the lab. The individual will then be given a drink that contains 75 grams of glucose. The individual then sits at the lab for two hours and then gives a second blood sample. The individual is generally required to remain in the lab for the entire duration of the test. Bringing a book or some other form of quiet entertainment is recommended.
This test helps to determine how an individual’s blood sugar levels change over time. When glucose is introduced to the body it moves quickly into the blood. In response to this increase in blood sugar, the pancreas starts to produce and release insulin into the bloodstream. The insulin helps the glucose to leave the blood stream and enter cells where it is stored for later energy use. Within two hours glucose in the blood should have fallen back to normal. In an individual with diabetes, however, the insulin is either not produced or is blocked from doing its job and the glucose remains in the blood.
Test results are interpreted by looking at the levels of glucose found in the blood. Glucose is measured in milligrams per deciliter (mg/dL). Normal results vary based of what test is being used. The following results are representative numbers based on the glucose tolerance test, 2 HR (2 Blood Specimens).
- After fasting: blood glucose levels above 95 mg/dL after fasting fall within the abnormally high range glucose levels.
- Glucose drink 2 hr: blood glucose levels above 155 mg/dL two hours after drinking 75 grams of glucose fall within the abnormally high range glucose levels.
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