How is diabetes diagnosed?

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If diabetes is suspected, your doctor will test your blood glucose levels for diabetes and prediabetes—a condition in which blood glucose levels are higher than normal but not elevated enough to be classified as diabetes. Your doctor should also check your blood pressure, body mass index (BMI) and cholesterol levels to provide youwith a fuller picture about your risk for developing diabetes.

Amy Campbell
Endocrinologist

Diabetes can be diagnosed in four ways. First, a fasting (no caloric intake for at least 8 hours) blood glucose can be checked. If the result is 126 mg/dl or higher, it's indicative of diabetes.

The second way to diagnose diabetes is by measuring blood glucose 2 hours after a 75 gram oral glucose tolerance test (OGTT). A result of 200 mg/dl or higher is indicative of diabetes.

The third way to diagnose diabetes is by the presence of classic symptoms of hyperglycemia (e.g., thirst, frequent urination) and a random blood glucose of 200 mg/dl or higher. And the fourth way to diagnose diabetes is by checking an A1C level. A level of 6.5 percent or higher is indicative of diabetes. In general, these tests should be repeated on a different day, unless the results are unequivocally high.

The most common way to diagnose diabetes in children and nonpregnant adults is through blood tests. These tests measure milligrams of blood sugar per deciliter, which is expressed by mg/dL. One test, the fasting blood glucose test, is usually performed after an eight-hour fast. If this test reveals a blood glucose level of 126 mg/dL or higher, diabetes is present. A random blood glucose test, taken at any time of the day, that indicates a blood glucose level of 200 mg/dL or higher also confirms diabetes, especially when accompanied by symptoms. Another test, called the oral glucose tolerance test, is conducted two hours after drinking 75 grams of glucose dissolved in water. If this test results in a blood glucose level of 200 mg/dL or higher, it is an indication of diabetes.

Medical experts now recommend a newer test, the glycated hemoglobin (A1C) test, to diagnose diabetes. The A1C test will measure average blood sugar level over a longer period of time. Certain conditions such as pregnancy can make these tests unreliable, so it is important to ask your doctor about which test is the right one for you.

Diabetes is diagnosed with a blood test. The most common type of diabetes is type 2 diabetes. It is diagnosed with one of several different blood tests.

The simplest test is a fasting blood sugar. This test simply measures the patient's blood sugar level when he is on an empty stomach. Another test that does not require a fast is called a Hemoglobin A1C test (also known as Glycohemoglobin). Less commonly, your doctor may decide to do a test called the glucose tolerance test. This test is done by having you drink a glucose (sugar) drink and then testing your blood sugar one or two hours later.

Type 1 diabetes is much less common and generally presents in childhood. It may be diagnosed with the same tests as type 2 diabetes, but most often it is diagnosed in the hospital when a patient becomes quite ill from the diabetes. It is very difficult to diagnose type 1 diabetes before significant symptoms develop.

Dr. Andrea Pennington, MD
Integrative Medicine Specialist

The blood test most commonly used in the evaluation of diabetes is the fasting plasma glucose test. The test is performed after an overnight or eight-hour fast during the day. Your blood is taken and sent to the lab to measure your glucose levels. The test results indicate whether your blood glucose level is normal, whether you have diabetes or whether you have impaired glucose tolerance, which we now refer to as prediabetes. Prediabetes is now more commonly used to emphasize the fact that without some lifestyle and nutrition intervention, the majority of people will go on to develop diabetes.

  • Normal: Normal blood sugar levels measure less than 100 milligrams per deciliter (mg/dl) after the fasting glucose test.
  • Prediabetes: Blood glucose levels of 100-125 mg/dl after an overnight or eight-hour fast is diagnosed as prediabetes. People with these results are considered to have impaired fasting glucose.
  • Diabetes: Diabetes is diagnosed when the blood glucose is 126 mg/dl or above.

Although your healthcare provider may suspect that you have diabetes because of your symptoms, the only sure way to tell is with blood tests. Blood tests are used to diagnose both type 1 and type 2 diabetes, as well as gestational diabetes.

The following blood tests are used for diagnosis:

A1C Test

  • The A1C test can be used to diagnose diabetes.
  • A1C values represent average blood glucose levels over the past 2–3 months.
  • Test measures the concentration of hemoglobin molecules that have glucose attached to them. The measure is given as a percentage. An 8 percent level means that 8 percent of your molecules are glycated (sugar coated).
  • An A1C of 6.5 percent or higher is used to diagnose diabetes.

Fasting Plasma Glucose Test

  • For this test, you will be asked not to eat or drink anything but water for at least 8–10 hours. Then, a sample of your blood is taken and the amount of glucose in the blood is measured.
  • For those without diabetes, the amount of glucose after fasting is usually less than 100 mg/dl.
  • However, when the amount of fasting plasma glucose is 126 mg/dl or higher, diabetes is suspected. A firm diagnosis of diabetes is made when two fasting plasma glucose tests, done on different days, are at least 126 mg/dl.

Random Plasma Glucose Test

  • This test measures the amount of plasma glucose at any given time and is done without fasting.
  • You may be diagnosed with diabetes if your plasma glucose is 200 mg/dl or higher and you have obvious symptoms, such as frequent urination, intense thirst, blurred vision, unexplained weight loss and extreme tiredness.

Oral Glucose Tolerance Test

  • For this test, you will be asked not to eat or drink anything overnight. Then, in the morning, a sample of your blood is taken before and two hours after you have a drink that contains glucose.
  • If your fasting plasma glucose is 126 mg/dl or higher and/or your post-drink plasma glucose is 200 mg/dl or higher, then you will be diagnosed with diabetes regardless of your symptoms.
Dana Artinyan
Nutrition & Dietetics Specialist

In order to have a diagnosis of diabetes, your fasting blood sugar needs to be greater than or equal to 126 mg/dL on two separate occasions.

Diabetes diagnosis is based on a combination of specific symptoms and glucose levels from lab results. In this video, Michelle Lalick, RN, BSN, CDE, of Mercy Health, shares more details about the symptoms used to diagnose diabetes.

Trinity Health is a Catholic health care organization that acts in accordance with the Catholic tradition and does not condone or support all practices covered in this site. In case of emergency call 911. This site is educational and not a substitute for professional medical advice, always seek the advice of a qualified healthcare provider.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.