Though not routinely used anymore, the oral glucose tolerance test (OGTT) is the gold standard for making the diagnosis of type 2 diabetes. However, it is still commonly used for diagnosing gestational diabetes.
With an oral glucose tolerance test, the person fasts overnight (at least eight but not more than 16 hours). Then, the fasting plasma glucose is tested. After this test, the person receives 75 grams of glucose (100 grams for pregnant women). There are several methods used by obstetricians to do this test, but the one described here is standard. Usually, the glucose is in a sweet-tasting liquid that the person drinks. Blood samples are taken up to four times to measure the blood glucose.
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For the glucose tolerance test to give reliable results, the person must be in good health (not have any other illnesses, not even a cold). Also, the person should be normally active (not lying down, for example, as an inpatient in a hospital) and should not be taking drugs that could affect the blood glucose. For three days before the test, the person should not eat a diet high in carbohydrates (150- 200 grams per day). The morning of the test, the person should not smoke or drink coffee.
What does the glucose tolerance test measure?
The classic oral glucose tolerance test measures blood glucose levels five times over a period of three hours. Some doctors simply get a beginning blood sample followed by a sample two hours after drinking the glucose solution. In a person without diabetes, the glucose levels rise and then fall quickly. In someone with diabetes, glucose levels rise higher than normal and fail to come back down as fast.
People with glucose levels between normal and diabetic have impaired glucose tolerance (IGT). People with IGT do not have diabetes. Each year, 1%-5% of people whose test results show IGT actually develop diabetes. Weight loss and exercise may help people with IGT return their glucose levels to normal. In addition, some doctors advocate the use of medications, such as metformin (Glucophage), to help prevent or delay the onset of overt diabetes. Recent studies have shown that IGT itself may be a risk factor for the development of heart disease, and whether IGT turns out to be an entity that deserves treatment itself is something that is currently debated.
How are the results of the glucose tolerance test evaluated?
Glucose tolerance tests may lead to one of the following diagnoses:
Normal response: A person is said to have a normal response when the 2-hour glucose level is less than 140 mg/dl, and all values between 0 and 2 hours are less than 200 mg/dl.
Impaired glucose tolerance: A person is said to have IGT when the fasting plasma glucose is less than 126 mg/dl and the 2-hour glucose level is between 140 and 199 mg/dl.
Diabetes: A person has diabetes when two diagnostic tests done on different days show that the blood glucose level is high.
Gestational diabetes: A woman has gestational diabetes when she has any two of the following: a 100g OGTT, a fasting plasma glucose of more than 95 mg/dl, a 1-hour glucose level of more than 180 mg/dl, a 2-hour glucose level of more than 155 mg/dl, or a 3-hour glucose level of more than 140 mg/dl.
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If the level is below 70 or you are experiencing symptoms such as shaking, sweating or difficulty thinking, you will need to raise the number immediately. A quick solution is to eat a few pieces of hard candy or 1 tablespoon of sugar or honey. Recheck your numbers again in 15 minutes to see if the number has gone up. If not, repeat the steps above or call your doctor.
People who experience hypoglycemia several times in a week should call their health care provider. It's important to monitor your levels each day so you can make sure your numbers are within the range. If you are pregnant always consult with your health care provider.
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However, it's important to continue to track your numbers so that you can make lifestyle changes if needed. If you are pregnant always consult with your physician.
Your level is high if this reading was taken before eating. Aim for 70-130 before meals and less than 180 two hours after meals.
Even if your number is high, it's not too late for you to take control of your health and lower your blood sugar.
One of the first steps is to monitor your levels each day. If you are pregnant always consult with your physician.
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