The Best Tests to Diagnose Diabetes


To promote the early detection of diabetes and reduce the risk of long-term complications, the American Diabetes Association (ADA) recommends that adults be initially screened for the disease at age 45, and if the results are normal, every three years thereafter. The U.S. Preventive Services Task Force (USPSTF), a federal panel of medical experts who establish national recommendations for preventive care, advises that people between ages 40 and 70 who are overweight (BMI 25 to 30) or obese (BMI above 30) should be screened every three years.

These guidelines may seem confusing, but keep in mind that they’re only recommendations. The decision to screen should also be based on other risk factors that can predispose you to prediabetes and diabetes. You can assess your risk by taking the online prediabetes screening quiz at, sponsored by the Centers for Disease Control and Prevention (CDC), the ADA, and other organizations. If the quiz results suggest you’re at elevated risk for prediabetes, print out your results and bring them with you to your next doctor’s visit. To diagnose prediabetes, your doctor will measure your blood glucose. If your results are in the prediabetes range, your doctor will recommend lifestyle interventions to help prevent you from progressing to full-blown diabetes.

Three blood tests are considered the most accurate for diagnosing prediabetes and diabetes:

  • Fasting plasma glucose test measures blood sugar level after an overnight fast (eight to 12 hours) and has been the standard initial diagnostic method for decades. The ADA considers prediabetes as a fasting glucose level of 100 to 125 milligrams per deciliter (mg/dL) of blood; the World Health Organization (WHO) recommends a slightly higher initial cutoff of 110 mg/dL for defining prediabetes. Type 2 diabetes is generally defined as a result of 126 mg/dL or higher on two fasting plasma glucose tests.
  • Hemoglobin A1c testreflects your blood sugar over the past two or three months. You don’t have to fast for this test. The ADA defines prediabetes as a hemoglobin A1c level of 5.7 to 6.4 percent; the International Expert Committee’s A1c level range for prediabetes is 6.0 to 6.4 percent. An A1c level of 6.5 percent or higher meets ADA criteria for the diagnosis of diabetes.

    This test measures the amount of glucose attached to hemoglobin-the oxygen-carrying protein in red blood cells that gives blood its color. The A1c test was originally used to monitor glucose levels in people already diagnosed with diabetes, but it is now also used to diagnose type 2 diabetes and prediabetes. As blood glucose levels rise, so does the amount of glucose attached to hemoglobin. Since hemoglobin circulates in the blood until the red blood cells die (half of red blood cells are replaced every 120 days), the A1c test measures average blood glucose levels over the previous two to three months.

    In general, if you have been diagnosed with diabetes, the ADA recommends keeping your A1c levels at about 7 percent. Your doctor may want you to aim for a different target, depending on your age, treatment, and other factors.

  • Oral two-hour glucose tolerance test first measures your blood sugar following an overnight fast, after which you drink a very sweet solution containing a standard amount of glucose. Your blood is tested again two hours later. This test is typically done when diabetes is suspected but you have normal or borderline results on a fasting plasma glucose test. The ADA considers an oral two-hour glucose tolerance test of 140 to 199 mg/dL as prediabetes, with 200 mg/dL or higher as diabetes.

If any of your test results come back as abnormal, your doctor will likely repeat the test to confirm a diagnosis.