What is the Glucose Tolerance Test?

So you think you may have hypoglycemia. You have all the symptoms. After discussing it with your physician, he agrees to give you a glucose tolerance test (GTT) to confirm the diagnosis. A test for three or four hours is requested when diabetes is suspected, but a six-hour glucose tolerance test is, by far, the most reliable method to detect low blood sugar. The HSF has always recommended that you settle for nothing less than the six-hour GTT. However, for a different perspective please see the section titled “Ask The Experts” for Dr. Baird’s response to the question of whether a patient should take a glucose tolerance test to confirm hypoglycemia.

The night before having the GTT, you will be asked to fast after your evening meal. You are to eat or drink nothing until the time of the test. When you arrive at the doctor’s office or laboratory, still fasting, a tube of blood will be drawn and you will be asked to give a urine specimen. Then you will be given a very sweet beverage called “Glucola” to drink. This drink contains a measured amount of glucose. Your blood will be drawn in 30 minutes and once again in one hour after drinking the Glucola. For each hour after that, you will give a blood sample until five or six hours have passed. A urine specimen is given each time your blood is drawn. Each tube of blood and each urine specimen are tested to determine the amount of glucose it contains. When the report is sent to your doctor, he or she will be looking for glucose levels above or below normal at any time during the test. During the test, you may start to sweat, get dizzy, weak or confused. If you experience these symptoms to the point of being extremely uncomfortable for you get a headache or your heart starts beating quickly, ask the doctor’s staff to draw your blood IMMEDIATELY. Any of those symptoms could be a sign that your blood sugar has dropped to a very low level, and you want your doctor to have the lowest readings possible. If you wait until the next hour, your blood sugar may go back up and your doctor will be deprived of information essential to making an accurate diagnosis.

The interpretation of the GTT is just as critical as its administration. Because individuals have different body chemistries, what is a normal drop or curve for one patient may not be for the next. Do not forget that laboratory tests are only aids to a diagnosis, not the final word. Remember, too, that the test is not for everyone. Children and the elderly, in particular, frequently require another method. Dr. Carlton Fredericks, author of Carlton Fredericks’ New Low Blood Sugar and You, frequently used “therapeutic diagnosis.” “This means putting the suspected hypoglycemic on the correct diet and watching the response. If, after a month or two, the symptoms are significantly reduced, the diagnosis has been established.” This procedure can be a less expensive, more convenient and less stressful method for diagnosing low blood sugar.

In conclusion, if you’ve read the basic facts about the glucose tolerance test, discussed it thoroughly with your physician and both of you have decided that this test is necessary, read the do’s and don’ts first.

Do understand the purpose, procedure and instructions BEFORE you have the glucose tolerance test administered.

Do make sure the test is scheduled in the morning (no later than 9:00 a.m.).

Do ask the doctor or nurse to repeat instructions if you do not fully comprehend what you are or are not supposed to do.

Do tell your physician, if he/she is not aware, if you are on any kind of medication. Some medications may affect blood sugar levels.

Do use the “therapeutic diagnosis” for children and the elderly.

Do bring someone with you, especially if you are experiencing severe symptoms.

Do bring a book, newspaper or magazine of your choice to help overcome the boredom. Sitting five or six hours is not something we’re used to doing. Consequently, restlessness often sets in.

Do have a pen and paper available to write down all the symptoms you are experiencing and at what time.

Do bring a sweater with you. Very often, a patient will experience chills during the GTT. It is best to be prepared.

Do arrange beforehand to have someone pick you up if you go alone for the test. Sometimes, afterward, you may be weak and driving could be difficult.

Do bring a snack to eat immediately after the test, particularly if you must go home alone. Eating some protein (nuts, seeds, meat, cheese, etc.) will bring your blood sugar up, allowing you to feel good enough to get home safely.

Do set up an appointment before you leave to go over your test results.

Don’t demand a glucose tolerance test. It is not always necessary.

Don’t accept a three- or four-hour glucose tolerance test for diagnosing hypoglycemia.

Don’t demand to have the glucose tolerance test if you have a fever or infection. It could affect the test results.

Don’t be shortchanged. Go over the results of your GTT with your physician thoroughly.

Don’t be fooled by the terms “borderline” or “mild” in the case of hypoglycemia. Too often when patients hear these terms, they don’t take their diagnosis seriously. This could eventually cause grave consequences.

Don’t dismiss the fact that you may still be hypoglycemic even if the GTT doesn’t confirm the diagnosis. Laboratory tests are not always conclusive. The conditions under which the test is given may alter the results. The best rule to follow is: don’t treat the results of the test, treat the symptoms.

ASK the Experts

Q. I’ve read so much about the glucose tolerance test (GTT), but I’m more confused than ever. Should I take it to confirm that I have hypoglycemia?

A. 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, which 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 (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. At this point, the patient should eat something appropriate 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 is 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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