How Is Type 2 Diabetes Diagnosed? 3 Key Tests to Know About

Experts recommend screening for diabetes starting at age 45.

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Testing for type 2 diabetes is crucial, since symptoms of type 2 diabetes often do not appear until blood sugar levels are already very high. That means you could live for months or years with type 2 diabetes and not even know it. It’s more common than you might think: one in four Americans with diabetes doesn’t actually know they have it, according to the Centers for Disease Control and Prevention.

“Patients with type 2 diabetes often don’t have any symptoms, and they’re diagnosed on routine blood testing,” says Sonal Chaudhry, MD, endocrinologist at NYU Langone Health.

Your doctor will want to check the following numbers to see whether you have type 2 diabetes.

  • Fasting blood glucose test: This number tests how much glucose is in the blood when you’re in a fasting state (usually at least eight hours), and this is included in routine blood tests. A fasting blood glucose equal to or greater than 126 may indicate type 2 diabetes, according to Minisha Sood, MD, endocrinologist in New York City.

  • A1C test: “This test measures your average blood sugar over the preceding two to three months,” says Dr. Chaudhry. An A1C level above 6.5 is consistent with type 2 diabetes. Learn more about the A1C test  here.

  • Glucose tolerance test: In this test, doctors will record your fasting blood glucose, and then have you drink 75 grams of a glucose drink. After two hours, you’ll take a blood glucose test again and see how much glucose is still in the blood—or how well your body “tolerates” glucose. After the second test, a number above 200 mg/dL indicates type 2 diabetes.

It’s possible for one test to suggest you have type 2 diabetes and another to show “normal” results. That’s why doctors may do repeat tests to get a more accurate picture. Different tests results may mean you are in an early stage of type 2 diabetes and blood glucose levels are not consistently high, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

“The American Diabetes Association recommends that all patients over the age of 45 are screen for type 2 diabetes,” says Dr. Chaudhry. “If that screening is normal, then they should be screened at least every three years for diabetes.” (Here are other tests women need in their 40s and tests women need in their 50s.)

Of course, people with risk factors of type 2 diabetes—such as obesity or a family history of diabetes—should likely be tested earlier and more frequently. If you’ve already been diagnosed with prediabetes, you should get blood glucose levels checked again every one to two years, according to the American Diabetes Association.

Don’t wait until you’ve noticed symptoms of diabetes, which may not show up until the disease has progressed to an advanced state. “The earlier you find out, the earlier you’re able to work with your full team in order to manage the disease,” says Nesochi Okeke-Igbokwe, MD, internist in New York City.

An early diagnosis of diabetes  has many benefits for your health, such as reducing the risk of serious complications. Learn how to protect your vision with diabetes, prevent foot problems with diabetes, and manage high and low blood sugar with diabetes.