The oral glucose tolerance test, or OGTT, screens for diabetes mellitus and gestational diabetes if you are pregnant or have symptoms of diabetes like excessive thirst and urination, night sweats and an elevated blood glucose. The OGTT evaluates carbohydrate metabolism following a dose of glucose and aids in confirming even mild diabetes and diet-controlled diabetes. Preparation for the test include following a test-specific diet and fasting, according to Lab Tests Online.
Test Preparation
Preparation for the 3-hour glucose test includes consuming a high-carbohydrate diet for three days and fasting for 10 to 16 hours before the test. Eating at least 150 to 200 grams of carbohydrates like fruits, vegetables, whole grains, and dairy products is included in preparations. Avoiding smoking, coffee, alcohol and strenuous exercise before and during the test is included in the instructions. The guidelines for testing pregnant women for gestational diabetes are less restrictive.
Procedure
The test requires five blood samples and, depending on the lab, five urine samples. Initially, a fasting blood sample is drawn, a urine specimen obtained if needed, followed by a dose of glucose. The amount of glucose is individualized based on your height, weight and body surface area. Blood samples are drawn after 30 minutes and then at one hour, two hour and three hour intervals, according to MedlinePlus.
Types
The OGTT may be the right test for you but there are two other tests that may be used to confirm OGTT results. The intravenous glucose tolerance test, or IVGTT, also measures glucose response and involves an infusion of 50 percent glucose solution, according to MedlinePlus. The cortisone glucose tolerance test, or CGTT, is used for borderline diabetics who have a family history of the disease and the OGTT results are normal.
Results
Normal glucose levels peak within 30 minutes to one hour of the oral glucose challenge. Glucose levels go back to fasting levels or lower within two to three hours according to the University of Michigan Health Systems. Lab Tests Online indicates normal urine specimen values show little to no evidence of excess glucose, called glycosuria, throughout the testing. If your values don't reflect the norms, your physician may suggest further tests.
Effects
Prior to the test, your instructions include an alert for the symptoms of low blood sugar or hypoglycemia. You may experience symptoms like weakness, sweating, restlessness, hunger and nervousness. Report these symptoms immediately to your physician as the test may need to be discontinued.
Interfering Components
If you are recovering from a recent illness like infection, fever or an acute illness like a heart attack, the effects may elevate your blood sugar. Pregnancy can also alter glucose levels. Noncompliance with diet and exercise restrictions may interfere with accurate results. Medications like high blood pressure drugs, insulin, diuretics, steroids, and oral contraceptives can also alter results. Your age is also a factor, since glucose tolerance increases after the age of 50.
