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How Is Glucose Tolerance Testing Used to Diagnose Diabetes?

  • Medical Author:

    Robert Ferry Jr., MD, is a U.S. board-certified Pediatric Endocrinologist. After taking his baccalaureate degree from Yale College, receiving his doctoral degree and residency training in pediatrics at University of Texas Health Science Center at San Antonio (UTHSCSA), he completed fellowship training in pediatric endocrinology at The Children's Hospital of Philadelphia.

  • Medical Editor: Melissa Conrad Stöppler, MD, Chief Medical Editor
    Melissa Conrad Stöppler, MD, Chief Medical Editor

    Melissa Conrad Stöppler, MD, Chief Medical Editor

    Melissa Conrad Stöppler, MD, is a U.S. board-certified Anatomic Pathologist with subspecialty training in the fields of Experimental and Molecular Pathology. Dr. Stöppler's educational background includes a BA with Highest Distinction from the University of Virginia and an MD from the University of North Carolina. She completed residency training in Anatomic Pathology at Georgetown University followed by subspecialty fellowship training in molecular diagnostics and experimental pathology.

Ask Glucose Tolerance Testing Diabetes Related Articles

Ask a Doctor

My specialist recently diagnosed me with type II diabetes. I want to be proactive and learn all I can about the disease. She used something called a “glucose tolerance test” to confirm her diagnosis. How does that test work?

Doctor's Response

The glucose tolerance test is just one of several common measures to diagnose diabetes, both type I and type II.

The health-care professional will take a history including information about the patient's symptoms, risk factors for diabetes, past medical problems, current medications, allergies to medications, family history of diabetes, or other medical problems such as high cholesterol or heart disease, and personal habits and lifestyle.

Laboratory tests that can confirm the diagnosis of diabetes are the following:

Finger stick blood glucose: This rapid screening test may be performed anywhere, including community-based screening programs.

  • Although not as accurate as testing blood in a hospital laboratory, a finger stick blood glucose test is easy to perform, and the result is available quickly.
  • The test involves sticking the patient's finger for a blood sample, which is then placed on a strip that has been inserted into a machine that reads the blood sugar level. These machines are only accurate to within about 10%-20% of true laboratory values.
  • Finger stick blood glucose values tend to be most inaccurate at very high or very low levels, so abnormally low or high results should be confirmed by repeat testing. Finger stick is the way most people with diabetes monitor their blood sugar levels at home.

Fasting plasma glucose: The patient will be asked to eat or drink nothing for eight hours before having blood drawn (usually first thing in the morning). If the blood glucose level is greater than or equal to 126 mg/dL (without eating anything) at any age, they probably have diabetes.

  • If the result is abnormal, the fasting plasma glucose test may be repeated on a different day to confirm the result. Or the patient may undergo an oral glucose tolerance test or a glycosylated hemoglobin test (often called "hemoglobin A1c") as a confirmatory test.
  • If fasting plasma glucose level is greater than 100 but less than 126 mg/dL, then the patient has what is called impaired fasting glucose, or IFG. This is considered to be prediabetes. These patients do not have diabetes, but they are at high risk of developing diabetes in the near future.

Oral glucose tolerance test: This test involves drawing blood for a fasting plasma glucose test, then drawing blood for a second glucose test at two hours after drinking a specific sweet drink (containing up to 75 grams of sugar).

  • If the blood sugar level after the sugar drink rises over or equal to 200 mg/dL, the patient has diabetes.
  • If the blood glucose level is between 140 and 199 mg/dL, then the patient has impaired glucose tolerance (IGT), also a prediabetic condition.

Glycosylated hemoglobin or hemoglobin A1c: This test measures how high the blood sugar levels have been over approximately the last 120 days (the average life span of the red blood cells on which the test is based).

  • Excess blood glucose hooks itself on to the hemoglobin in red blood cells and stays there for the remaining life of the red blood cell.
  • The percentage of hemoglobin that has had excess blood sugar attached to it can be measured in the blood. The test involves having a small amount of blood drawn or by finger stick.
  • The hemoglobin A1c test is the best measurement of blood sugar control in people known to have diabetes. The normal value is under 6%. Hemoglobin A1c levels of 7% or less indicate good glucose control. A result of 8% or higher indicates that blood sugar levels are too high, too often.
  • The hemoglobin A1c test is the best test for diabetes follow-up care. Although less than ideal for diagnosing diabetes, hemoglobin A1c above 6% is highly suggestive of diabetes. Generally, another confirmatory test would be needed to diagnose diabetes.
  • The hemoglobin A1c test is usually measured about every three to six months for people with diabetes, although it may be done more frequently for people who are having difficulty achieving and maintaining good blood sugar control.
  • This test is not used for people who do not have diabetes or are not at increased risk of diabetes.
  • Normal values may vary from laboratory to laboratory, although an effort is under way to standardize how measurements are performed.

For more information, read our full medical article on diabetes.

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Reviewed on 2/23/2018
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