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Blood Tests for Diabetes: Oral Glucose Tolerance Test

This test involves drinking a glucose solution and checking how well your body breaks down sugar.


Also known as: OGTT; 2-hour glucose tolerance test.

What is it? A test that measures blood glucose levels before and 2 hours after you drink an oral dose of glucose solution (75 to 100 grams of an extremely sweet drink), which should cause glucose levels to rise in the first hour, then fall back to normal within two hours as the body produces insulin to normalize glucose levels. Test results show how well your body is able to process glucose. 

Why is this test performed? To confirm a diagnosis of diabetes or gestational diabetes (and to diagnose other metabolic diseases). Because the OGTT is a more sensitive test than the fasting plasma glucose test, and involves multiple blood draws to monitor insulin production, it can often detect cases of diabetes that may be missed by the fasting test.

How is the oral glucose tolerance test performed? The OGTT is typically performed in the morning, after you’ve fasted for at least eight hours. Your blood will be drawn to measure your fasting blood sugar level. You will then be given the sugary solution to drink, and your blood sugar levels will be tested periodically over the next two hours.

How frequently should this test be performed? Once, in select individuals, for diagnosis of diabetes. A second retest may be appropriate in some patients, particularly pregnant women being tested for gestational diabetes. Additional laboratory tests may be required for an accurate diagnosis. 

What is the “normal” range for results? While laboratory test methods and measurements can vary for this test, normal OGTT results in those who are not pregnant are as follows:

  • Fasting: 60 mg/dl to 99 mg/dl (3.3 mmol/l to 5.5 mmol/l)
  • 1 hour: Less than 200 mg/dl (11.1 mmol/l)
  • 2 hours: Less than 140 mg/dl (7.8 mmol/l)

Factors such as age, weight, and race can influence results, as can recent illnesses and certain medications. For example, older individuals will have an upper-limit increase of 1 mg/dl in glucose tolerance for every year over age 50. For more information, ask your doctor which protocol your lab uses.

What do abnormal results mean?

For non-pregnant people:

  • A 2-hour blood sugar reading between 140 mg/dl and 199 mg/dl (7.8 mmol/l and 11.0 mmol/l) indicates prediabetes (impaired glucose tolerance).
  • A 2-hour blood sugar reading of 200 mg/dl (11.1 or higher) or higher indicates diabetes.

You may need to retake the test on another day to confirm diagnosis of diabetes.

If you’re taking the OGTT to test for gestational diabetes, abnormal results can differ, depending upon the size of the glucose drink and how often your blood sugar is tested.

Abnormal results for a 2-hour 75-gram OGTT:

  • Fasting: greater than 92 mg/dl (5.1 mmol/l)
  • 1 hour: greater than 180 mg/dl (10.0 mmol/l)
  • 2 hours: greater than 153 mg/dl (8.5 mmol/l)

Abnormal results for a 3-hour 100-gram OGTT:

  • Fasting: greater than 95 mg/dl (5.3 mmol/l)
  • 1 hour: greater than 180 mg/dl (10.0 mmol/l)
  • 2 hours: greater than 155 mg/dl (8.6 mmol/l)
  • 3 hours: greater than 140 mg/dl (7.8 mmol/l)

If fasting glucose levels peak below normal levels (i.e., approximately 140 mg/dl or 7.78 mmol/l), a condition that indicates increased glucose tolerance, such as Addison’s disease, hypothyroidism, malabsorption, or hypopituitarism, may be present.

SOURCES

American Diabetes Association. “Diagnosing Diabetes and Learning About Prediabetes.” September 22, 2014. Accessed August 30, 2017. http://www.diabetes.org/diabetes-basics/diagnosis/?referrer=https://www.google.com/

Centers for Disease Control and Prevention. “Getting Tested.” August 4, 2017. Accessed August 30, 2017. https://www.cdc.gov/diabetes/basics/getting-tested.html

Mayo Clinic. “Tests and Diagnosis.” July 31, 2014. Accessed August 30, 2017. http://www.mayoclinic.org/diseases-conditions/diabetes/basics/tests-diagnosis/con-20033091

National Institute of Diabetes and Digestive and Kidney Diseases. “Diabetes Tests & Diagnosis.” November 2016. Accessed August 30, 2017. https://www.niddk.nih.gov/health-information/diabetes/overview/tests-diagnosis

U.S. National Library of Medicine. “Glucose Screening Tests During Pregnancy.” MedlinePlus. Accessed August 30, 2017. https://medlineplus.gov/ency/article/007562.htm

U.S. National Library of Medicine. “Glucose Tolerance Test—Non-Pregnant.” Accessed August 30, 2017. https://medlineplus.gov/ency/article/003466.htm

Blood Tests for Diabetes: Oral Glucose Tolerance Test
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