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Screening and Diagnosis for Diabetes

Learn why you need a blood sugar test to accurately diagnose diabetes.

Medically reviewed in June 2022

Diabetes is easy to prevent—if it's caught early enough. It's also the seventh leading cause of death in the United States, a fact that should motivate every at-risk adult to put diabetes screening at the top of his or her stay-healthy to-do list.

When and how often should you get tested for diabetes? Current guidelines differ, but the American Diabetes Association (ADA) recommends all adults who are overweight or have other diabetes risk factors be screened for prediabetes and type 2 diabetes starting at age 45. Your doctor may still recommend screening even if you're at a normal weight but 45 or older.

Regular blood sugar tests are equally important for people already diagnosed with diabetes. These tests, performed both at home and the doctor's office, tell you how well you're controlling your blood sugar—a prerequisite to living a healthy, younger life with diabetes. Ask your doctor how often you should test your blood sugar.

Several types of tests can check for prediabetes, type 2 diabetes, and type 1 diabetes. All of them measure your blood glucose (sugar) level but under different circumstances.

Fasting Plasma Glucose (FPG) Test
This test measures your fasting blood sugar levels and is the preferred test for diagnosing diabetes. You'll be asked to fast overnight and give a blood sample in the morning before you've eaten anything. This test is used to diagnose prediabetes, type 2 diabetes, and type 1 diabetes.

What test results mean:

  • Normal: Lower than 100 mg/dL
  • Prediabetes: 100 to 125 mg/dL
  • Diabetes: Higher than 126 mg/dL

Oral Glucose Tolerance Test (OGTT)
This test measures how your body responds to a sugar "challenge." After fasting overnight, your fasting blood glucose is tested. You'll then be given a sugar solution to drink and your blood sugar will be tested over the next few hours. This test is used to diagnose all types of diabetes.

What the test results mean:

  • Normal: Less than 140 mg/dL
  • Prediabetes:141 to 199 mg/dL
  • Diabetes: Higher than 200 mg/dL

Hemoglobin A1c (HbA1c) Test
Unlike other tests, which measure blood sugar levels at a specific time, this test measures your average blood sugar level over the past 3 months. Take this test every 6 months if your diabetes is well-controlled, and every 3 months if it's poorly controlled.

What the test results mean:

  • Normal: Below 5.6%
  • High risk: 5.7% to 6.4% (the A1C is normally used to monitor, not diagnose, prediabetes
  • Diabetes: 6.5% or above

To confirm a diagnosis of diabetes, a repeat test must be done on a different day. A second test is not required for a diagnosis of prediabetes unless you're considering treatment with medication. Medication for prediabetes is not recommended without a confirmed diagnosis of both impaired glucose tolerance (IGT) and impaired fasting glucose (IFG).

random blood sugar test may be used to diagnose diabetes if there are also distinct symptoms that strongly suggest diabetes. A random glucose test may be given at any time, whether or not you've recently eaten, and a result of 200 mg/dL or higher indicates diabetes.

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