The pancreas produces a hormone known as insulin. It works by converting sugar, starches, and other foods into energy. If you're diabetic, your body is unable to produce insulin or properly use it. Without insulin, glucose cannot be stored or used, so it builds up in your bloodstream and raises your blood sugar. Performing diabetes diagnostic tests, like fasting plasma glucose and oral glucose tolerance tests, can provide valuable information for those who suspect they're at risk for diabetes.

Fasting Plasma Glucose Test

The fasting plasma glucose test (FPG) is the preferred diagnostic test for diabetes. A FPG test measures the effectiveness of insulin on blood glucose. A person with a normal blood glucose level will receive a reading between 70 mg/dl and 99 mg/dl. Those at risk for prediabetes may have a slightly higher reading of 100 mg/dl to 126 mg/dl. If you're diagnosed with prediabetes, your blood sugar is higher than normal; however, it's not as high as those with diabetes. Unfortunately, a reading above 126 mg/dl may be a sign of diabetes.[1]

The preparation for a FPG test is fairly simple, as it only requires patients to fast 12 to 14 hours before the test. Your doctor will then take a blood sample and examine its blood glucose content. A high reading doesn't necessarily mean prediabetes or diabetes. To warrant a diagnosis, the patient must be tested twice on two separate days and receive a high reading for both tests.

Oral Glucose Tolerance Test

The oral glucose tolerance test (OGTT) is another diagnostic test for diabetes. It is a more sensitive test and therefore often considered a better diabetes diagnostic test to identify the existence of a prediabetes condition. By evaluating how well the body metabolizes glucose, an OGTT can also diagnose gestational diabetes or diabetes. After a meal, a normal glucose level should be less than 140 mg/dl; however, a diabetic's blood glucose may be above 200 mg/dl. Although a reading of 140 mg/dl to 199 mg/dl doesn't indicate diabetes, it may mean you're at risk for prediabetes.[2]

The diabetes diagnostic criteria for an OGTT are different than a FPG test. It's important to eat a balanced meal prior to the test; however, OGTT patients should not eat or drink 8 hours before. Once you finish fasting, your doctor will take a blood sample. You'll then consume a beverage high in sugar so that the doctor can then draw more blood to test its glucose content. If your blood sugar remains high after 1 to 2 hours, you may have diabetes. Similar to an FPG test, an OGTT must be repeated in a later test to confirm a diagnosis.

If left unmanaged, diabetes can be life-threatening. However, with the help diagnostic tests for diabetes, you can start treating the disease. A glucose test should not be the only diagnostic criteria for diabetes. To ensure accuracy, you also need to receive a physical examination. During the examination, your doctor will note any symptoms like fatigue, frequent urination, and thirst. If you're diagnosed with prediabetes or diabetes, it's important to discuss the next steps with your physician. He/she can provide you with additional information regarding medication, diet, and exercise.