What Is a Glucose Intolerance Test?

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  • Written By: Lee Johnson
  • Edited By: Nancy Fann-Im
  • Last Modified Date: 02 November 2018
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A glucose intolerance test is used to determine a patient’s level of tolerance to glucose, in order to diagnose conditions such as type 2 diabetes. The two most common glucose intolerance tests are the fasting glucose test and the oral glucose tolerance test. Generally, these tests will be used together to determine whether or not a patient has diabetes.

The fasting glucose test is done after the patient hasn’t eaten anything for eight to ten hours, after which the patient’s blood is taken to measure the levels of glucose in his blood. A glucose-rich drink can then be provided and the oral glucose tolerance test taken two hours afterward. Generally, the tests are performed by taking a small sample of blood from the patient and placing it onto an indicator strip, which is inserted into a small machine designed to test glucose levels. Further readings are then taken at 30-minute to one-hour intervals after the glucose drink is administered, usually concluding after two hours. A glucose intolerance test can take up to three hours.


Glucose intolerance tests are usually performed to determine whether or not a patient has diabetes. Diabetes is a condition in which the patient’s body does not produce enough insulin, which is used as a biochemical bridge for glucose to get into the body’s cells, thereby producing energy. Insulin is produced by the pancreas, and if it isn’t produced or doesn’t function properly, excess glucose builds up in the blood. Pregnant women can develop gestational diabetes in the later stages of pregnancy.

This buildup of glucose, technically referred to as hyperglycemia, is what is tested in a glucose intolerance test. These tests are designed to find patients who have either an impaired fasting glucose or impaired glucose tolerance. Generally, the tests will be administered when a patient has symptoms of hyperglycemia, such as frequent urination, increased thirst, tiredness and blurred vision. They are also often performed when a patient is particularly at risk of developing diabetes as a result of diabetes in his immediate family, or when a person is overweight and over the age of 45.

Anything over 7 millimoles per liter (mmol/L) is considered to be indicative of diabetes on a fasting glucose intolerance test. On the oral glucose tolerance test, anything over 11.1 mmol/L is considered to show that the patient probably has diabetes. On some occasions, glucose will be injected directly into the blood for a glucose intolerance test.



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