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Glucose Tolerance Test

Alternate Names

  • oral glucose tolerance test
  • OGTT
  • GTT

Definition

This test is primarily used to measure how well the body can use, or metabolize, glucose. Glucose is the main form of sugar in the body.

Who is a candidate for the test?

The test is done when a doctor thinks a person has diabetes mellitus, which results in a high blood sugar level. It is also advised in all pregnant women to screen for diabetes of pregnancy.

How is the test performed?

In the glucose tolerance test, samples of a person's blood are measured at certain times after the person drinks a solution of glucose in water. A blood sample is first taken before the person drinks a sweet drink. Then, samples are taken again at certain times after drinking it.
Common times that are used include:
  • 30 minutes
  • 1 hour
  • 2 hours
  • 3 hours
To measure glucose in the blood, a sample of blood is needed. This is most often drawn from a vein in the forearm. To do this, the skin over the vein is first cleaned with an antiseptic. Next, a rubber tube called a tourniquet is wrapped around the upper arm. This enlarges the veins in the lower arm and restricts blood flow through them.
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. It is then sent to the laboratory for 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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.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, that is, not at the beginning and not at the end of urination, are collected at the same time intervals as the blood samples are taken.

What is involved in preparation for the test?

People are often advised not to eat for at least 8 hours before the test. This is called fasting. Certain medicines, such as those for diabetes, may be stopped before the test. A person should check with the healthcare professional for specific instructions.

What do the test results mean?

Normal fasting blood sugar (FBS) levels are less than 100 milligrams/deciliter, or mg/dL. If a person does not have diabetes, the glucose levels will rise and then fall quickly after drinking the sweet liquid. When a person has diabetes, glucose levels will rise higher and fail to come down as fast as those in a person without diabetes.
Healthy blood sugar levels after drinking the glucose are less than 140 mg/dL. If the blood sugar level is 140 to 199 mg/dL 2 hours after drinking the liquid, a diagnosis of prediabetes can be made. If the blood sugar level is higher than 200 mg/dL, then another test is done on a different day to confirm whether the person has diabetes.
If levels are lower than normal, the person may have:
  • hypoglycemia, or low blood sugar
  • bowel problems that interfere with absorbing the glucose into the body
  • certain hormone imbalances, such as a low thyroid hormone level
  • a rare tumor that causes the pancreas to make too much insulin
  • certain substances in his or her body that interfere with the test, such as caffeine
If levels are higher than normal, the person may have:
  • prediabetes
  • diabetes mellitus
  • gestational diabetes
  • hormone imbalances. These include a high thyroid hormone level or a high level of cortisol, which is a hormone that is important in metabolism.
  • damage to the pancreas, which is the 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. For example, this may include tumors that secrete adrenaline, a stress hormone.
  • certain medicines in his or her system that interfere with the test. This may include corticosteroids or steroids.

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