Diagnosing Diabetes Using the Oral Glucose Tolerance Test
written by: Melissa Murfin
• edited by: Diana Cooper
• updated: 5/24/2011
Understanding how the oral glucose tolerance test is used to diagnose diabetes can help patients better comprehend their diagnosis and goals for treatment.
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Diabetes mellitus occurs when the pancreas produces insufficient amounts of insulin or the body is unable to properly use the insulin to process glucose. Levels of glucose increase in the blood and urine which indicate that not enough sugar is being properly utilized for energy. This increased glucose in the blood can lead to complications such as blindness, kidney failure, heart disease and nerve damage.
It is possible to measure glucose in both blood and urine. One method of diagnosing diabetes is by measuring the level of glucose in the blood after not eating for roughly 8 to 12 hours. This fasting level of glucose in the blood is considered to be in the prediabetes range if it is greater than 100 but less than 126. Diabetes is actually diagnosed when this fasting level reaches 126 or higher. A diagnosis is not made until the same results come back from two tests performed on two separate days.
However, fasting blood sugar doesn't tell the whole story. Some people may have normal fasting blood sugar, but have elevated levels after meals. In this case, an oral glucose tolerance test can help diagnose diabetes. Diagnostic criteria with oral glucose tolerance test have been defined by the American Diabetes Association.
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The Oral Glucose Tolerance Test (OGTT)
The OGTT has been used for many years to diagnose diabetes. Patients go to the lab fasting, at which time their initial blood glucose level is measured. They then drink a beverage similar to a flat soda which contains about 75 grams of glucose. Blood levels are drawn at one-hour and two-hours after drinking the beverage and glucose is measured. Diagnostic criteria with oral glucose tolerance test indicate that a patient has diabetes when their two-hour blood glucose level is > 200 mg/dL. Patients who are at risk for diabetes will have a two-hour level ranging from 140 mg/dL to 199 mg/dL.
Diabetes during pregnancy is also a concerned and can be diagnosed with an OGTT. Women can be screened for gestational diabetes between 24 and 28 weeks of pregnancy. Diagnostic criteria with the oral glucose tolerance test are slightly different during pregnancy. The American Diabetes Association defines diabetes during pregnancy when blood glucose levels during OGTT are:
fasting > 92 mg/dL
one-hour > 180 mg/dL
two-hour > 153 mg/dL
The World Health Organization (WHO) also has very similar diagnostic criteria for OGTT during pregnancy, with fasting glucose > 95 mg/dL and two-hour > 155 mg/dL considered to be in the range for diabetes. The WHO also has diagnostic criteria for a 100 g glucose tolerance test that can be used in pregnancy.
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Discussing the OGTT with Your Health Care Provider
People who are concerned about diabetes should discuss any questions with their healthcare provider who can order proper testing. Should an OGTT be required, this information will certainly be considered by the provider when determining the patient's proper diagnosis. Helpful information for patients that includes diagnostic criteria with oral glucose tolerance test can be found on the American Diabetes Association's website.