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Glucose Tolerance Test

The goal of blood glucose tests is to find out whether you have a very large amount of glucose in your blood. Glucose Tolerance Tests (GTTs) are diagnostic tests designed to confirm a diagnosis that is suspected from a patient's symptoms. Venous blood is drawn in the laboratory and analyzed for its' glucose content. Different types of GTTs are used to diagnose diabetes-fasting plasma glucose, random plasma glucose, and oral glucose tolerance tests.

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When you get your test results, ask your doctor to explain them to you. Comparing your test results with those of family or friends may confuse or alarm you. You may not have had the same type of test, so your results could have a completely different meaning.

Fasting Plasma Glucose Test

The Fasting Plasma Glucose Test is provided daily, Monday through Friday, except holidays and is the preferred way to diagnose diabetes. Patients must be in the laboratory prior to 1500 hrs with active orders for a fasting glucose. After you have fasted overnight (at least 8 hours), a single sample of your blood is drawn. During the fasting period, no food is to be taken, no coffee or other beverages - only water is allowed. Normal fasting plasma glucose levels are less than 110 milligrams per deciliter (mg/dL). Fasting plasma glucose levels of more than 126 mg/dl on two or more tests on different days indicate diabetes.

Random Plasma Glucose Test

The Random Plasma Glucose Test is provided daily, Monday through Friday, except holidays. Patients must be in the laboratory prior to 1500 hrs with active orders for a random plasma glucose level. A random plasma glucose >200 mg/dl is indicative of diabetes. Diagnosis is established by repeat testing on a subsequent day. (A fasting glucose specimen is suggested.)

Oral Glucose Tolerance Test (GTT)

The One Hour GTT with a 50 gram Glucose Challenge (The O'Sullivan Screen)

The O'Sullivan screen is a screening test for women in their third trimester of pregnancy who have had no previous history of gestational diabetes/glucose intolerance, or pregnant women at any stage of pregnancy with either a prior history of gestational diabetes or strong clinical suspicion of developing this disorder. The test is administered from 0630 to 1500 hrs, Monday through Friday, except holidays. You do not need to be fasting, but you must remain in the lab for the duration of the test. You will be given a full bottle of Fisherbrand Glucose Tolerance Test Beverage (50g dose) orally over 5 minutes. One hour from the time you finish the glucose beverage, she will have her blood drawn and this will conclude the test.

If the O'Sullivan screen is between 130 mg/dl and 200 mg/dl, the 3-hr 100 gm or the 2-hr 75 gm GTT will be scheduled when requested by your physician. If the O'Sullivan screen glucose is greater than 200 mg/dl, a 2-hr or 3-hr GTT is generally not indicated, as this is nearly diagnostic for gestational diabetes. Generally, a fasting blood glucose is ordered as a follow-up/confirmatory test. In such cases, scheduling of the test must be approved by the Medical Director of Chemistry or another pathologist.

The Two Hour GTT with 75 gram Glucose Challenge

The oral GTT (2 hrs) is provided daily, Monday through Friday, except holidays. Patients must be in the laboratory prior to 1300 hrs. You will be instructed to follow a diet of 150 grams of carbohydrate per day for three days and fast for at least 8 hrs but not more than 14 hrs immediately prior to the oral GTT. Medications known to interfere with glucose metabolism should be discontinued if possible. In general, a GTT should not be performed on patients with acute illnesses, or hospitalized or inactive patients.

NOTE: Pregnant patients are not required to follow the 150 gm of carbohydrate diet prior to the oral GTT at the discretion of the attending physician.

For 3 days before the test you should have eaten a diet high in carbohydrates. You can obtain a menu outlining a 150 gm per day carbohydrate diet from the laboratory. Patients with small children will be asked not to bring the children with them (unless accompanied by another adult) on the day of the test. On the day of the test, a fasting glucose is drawn. The fasting sample will be taken directly to the STAT Lab. You will be instructed to have a seat and wait until called to start the test. If the fasting glucose is greater than 140 mg/dl, your doctor will be notified and the GTT will not be started. If the fasting glucose is less than or equal to 140 mg/dl, the GTT will be started.

You will be given a full bottle of Fisherbrand Glucose Tolerance Test Beverage (75g dose). Children are given 1.75g glucose/kg of body weight (not to exceed 75g). The glucose beverage should be taken orally over a period not to exceed 5 minutes. Once you have finished drinking the glucose beverage, the time will be noted. For the standard test, blood will be Drawn 2 hrs from the time the beverage is finished. (For example; if you finish drinking the beverage at 0813 hrs, blood will be drawn at 1013 hrs.) During the GTT, the you will remain in the laboratory so first aid can be administered if there are any adverse symptoms. You should refrain from any other food or beverage intake during the test period. Refrain from smoking and remain seated in the laboratory area (avoid physical exertion or anxiety). The results are entered into the computer system and interpretation is made by your physician.


The Three Hour GTT with 100 gram Glucose Challenge

The oral GTT (3 hrs) is provided daily, Monday through Friday, except holidays. Patients must be in the laboratory prior to 1100 hrs. This test is for pregnant patients only. You will have fasted for at least 8 hrs but not more than 14 hrs immediately prior to the oral GTT. Medications known to interfere
with glucose metabolism should be discontinued if possible. In general, a GTT should not be performed if you have an acute illnesses, or are hospitalized or inactive. Patients with small children will be asked not to bring the children with them (unless accompanied by another adult) on the day of the test. On the day of the test, a fasting glucose is drawn. The fasting sample will be taken directly to the STAT Lab. You will be instructed to have a seat and wait until called to start the test. If the fasting glucose is greater than 140 mg/dl, your doctor will be notified and the GTT will not be started. If the fasting glucose is less than or equal to 140 mg/dl, the GTT will be started. You will be given a full bottle of Fisherbrand Glucose Tolerance Test Beverage (100G dose). The glucose beverage should be taken orally over a period not to exceed 5 minutes. Once you have finished drinking the glucose beverage, the time will be noted. For the standard test, blood will be drawn 1 hour, 2 hrs and 3 hrs from the time the beverage is finished. (For example; if you finish drinking the beverage at 0813hrs, your blood will be drawn at 0913 hrs, 1013 hrs and 1113 hrs. During the GTT, the patient should remain in the laboratory so first aid can be administered if there are any adverse symptoms. The results are entered into the computer system and interpretation is made by your physician.


(Reference: The American Diabetes Association, Clinical Practice Recommendations)