What is the Glucose Tolerance Test (GTT)?

I’ve read so much about the Glucose Tolerance Test (GTT). I’m more confused than ever. Should I take it to confirm I have hypoglycemia?

Doctors who have significant experience with blood sugar management disorders such as hypoglycemia are able to identify probable candidates for this diagnosis through symptoms, history and examination alone. Subjecting a patient to a glucose tolerance test can be very stressful and many doctors opt not to do these tests for that reason.

Additionally, the GTT may not provide enough information to establish the diagnosis that could further confuse and complicate the situation. A single finger prick seldom tells us enough to be of significant value. It is both the absolute level of blood sugar, as well as the change in levels, that assist us in making a diagnosis.

Also, the standard glucose tolerance test, due to its lack of flexibility, is prone to error and can easily miss some of the low blood sugar readings and precipitous drops in glucose levels as the patient responds to a heavy glucose load.

If a patient’s symptoms warrant it, I use a different protocol for the GTT. This enhanced design remedies the shortcomings of the standard test and has in practice identified a higher percentage of patients with blood sugar management disorders. For the purposes of this test, the patient is instructed to eat a diet high in carbohydrates for three days prior to the test. On the day of the test the patient is to fast from midnight on. Water is permissible.

The first (venous blood, not finger stick) serves as a baseline for both blood glucose and serum C-peptide insulin. Insulin levels are monitored along with blood glucose measurements. The test proceeds according to the standard protocol until the patient begins to become symptomatic. With the onset of symptoms of falling or low blood sugar, blood samples are drawn every fifteen minutes and recorded until stabilizing around baseline level. At this time, the test can be terminated. The patient should eat something appropriate at this point and should not be released from the examination center until they are perceived to be in control of their faculties. Any examination facility performing this testing should be equipped to manage hypoglycemia convulsions. If the fasting blood sugar level is in excess of 300mg/dl, the test should not be performed.

This test is considered positive for hypoglycemia if the rate of glucose drop is greater than 100 mg/dl/hr or an absolute blood sugar level less than 60 mg/dl at any time during the test. Whether or not you should have a glucose tolerance test should be determined by you and your physician. — Dr. Douglas M. Baird

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