Special Tests
The Glucose Tolerance Test and the Glycosylated Hemoglobin Test
Oral Glucose Tolerance Test
This test is primarily used to measure how well the body can metabolize glucose. Glucose, as we know, is the main form of sugar in the body.
The test is performed on a person who is suspected of having Diabetes Mellitus, or high blood sugar. It is also advised that all pregnant women take this test, to screen them for Gestational Diabetes.
The Test Method
In the Glucose Tolerance Test, samples of a person's blood are measured at specified intervals after the person drinks a solution of glucose in water.
A blood sample is first taken before the person drinks the glucose solution. Samples are taken again at certain intervals after the person swallows the solution. Common times used include 30 minutes, 1 hour, 2 hours, and 3 hours after the swallowing of the glucose solution.
To measure glucose in the blood, a sample of blood is needed. This is usually drawn from a vein in the forearm or hand.
Step 1:
For taking a measure of glucose in the blood, the skin over the vein is first cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them.
Step 2:
A very thin needle is gently inserted into a vein and the tourniquet is removed. Blood flows from the vein through the needle and is collected in a syringe or vial.
Step 3:
The sample is then sent to the laboratory for glucose testing. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
In some cases, urine samples may also be used to look for extra glucose. Samples of midstream urine are collected at the same time intervals as the blood samples are taken.
Preparation for the Test
People undergoing the test are often advised not to eat for at least 8 hours before the test. Certain medications, such as diabetes medications, may be stopped before the test.
Interpreting Test Results
The normal value of blood glucose after fasting for 8 hours is 70 to 110 milligrams/deciliter, or mg/dl. Various blood glucose levels are considered normal at each time interval after swallowing the glucose. This test is normally done to test for diabetes; however, other conditions can cause abnormal results.
If levels are higher than normal, the person being tested may have:
- Diabetes Mellitus, which occurs when blood sugar is is too high. Some cases of Diabetes are permanent, whereas others are related to a pregnancy and disappear after the delivery of the child.
- Hormonal imbalances. These include a high thyroid hormone level or a high level of cortisol, an important hormone used in metabolism.
- Damaged pancreas. The pancreas is an organ that secretes insulin. Insulin is a hormone that helps keep the blood sugar from getting too high. When damaged, the pancreas may not be able to secrete enough insulin.
- Certain tumors, such as ones that secrete adrenaline, a stress hormone.
- Certain drugs in their system that interfere with the test, such as corticosteroids, or steroids.
If levels are lower than normal, the person may have:
- Hypoglycemia, or low blood sugar
- Bowel problems that interfere with absorption of glucose into the body.
- Certain hormonal imbalances, such as a low thyroid hormone level.
- A tumor that secretes too much insulin. This is a rare condition.
- Certain drugs in the system that interfere with the test, such as caffeine.
Diagnosing diabetes with Glucose tolerance tests
Glucose tolerance tests may lead to one of the following diagnoses.
Normal response: A person is said to have a normal response when the 2-hour glucose level is less than 140 mg/dl, and all values between 0 and 2 hours are less than 200 mg/dl.
Impaired glucose tolerance: A person is said to have IGT when the fasting plasma glucose is less than 126 mg/dl and the 2-hour glucose level is between 140 and 199 mg/dl.
Diabetes: A person has Diabetes when two diagnostic tests done on different days show that the blood glucose level in the body is high.
Gestational diabetes: A woman has gestational diabetes when she has any two of the following: a fasting plasma glucose of more than 95 mg/dl, a 1-hour glucose level of more than 180 mg/dl, a 2-hour glucose level of more than 155 mg/dl, or a 3-hour glucose level of more than 140 mg/dl.
Doctors need to use tests and test results to diagnose diabetes. But test results are just part of the information that goes into the diagnosis of the disorder. Doctors also take into account physical conditions, symptoms and the medical history of the patient to reach a conclusion.
Glycosylated Hemoglobin
Glycosylated hemoglobin, or Hgb AlC, is a test that reflects the average of your blood sugar levels over the past 2-3 months.
Hemoglobin is a protein found inside red blood cells. Glucose binds to hemoglobin, causing it to become glycosylated. The higher the percentage of hemoglobin that is glycosylated, the higher your average blood sugar for the past 2-3 months.
The GHb test requires only one sample of blood, which can be taken at any time of the day, even right after a meal.
Although a high GHb almost always means IGT or diabetes, people with IGT or diabetes can have normal GHb levels. So the GHb test is not used to diagnose diabetes. Rather it is used to monitor blood glucose control.
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