Diagnosing Diabetes

Here are federal guides for when to get checked for diabetes:

• People between ages 35 and 70 who have overweight or obesity should be tested for prediabetes or diabetes. If the first blood sugar (glucose) test is normal, they should get tested again every 3 years.

• People of any age should be tested every 3 years for prediabetes or diabetes if they are overweight or obese (body mass index of 25 or more or 23 or more in Asian Americans) and have one or more risk factors. Risk factors include:

  • Have a mother, father, brother, or sister with diabetes

  • Are part of a high-risk ethnic group. This means African American, Hispanic, Asian, Pacific Islander, Native Alaskan, or Native American.

  • Had heart and blood vessel disease in the past

  • Have blood pressure at 140/90 mm/Hg or higher. Or are taking medicine for high blood pressure.

  • Have blood fat levels that are not normal. For example, high-density lipoproteins of less than 35 mg/dL. Or triglycerides greater than 250 mg/dL.

  • Are not active

  • Had impaired glucose tolerance during a past test for diabetes

  • A history of diabetes during pregnancy that went away after delivery

  • Are a woman with polycystic ovarian syndrome

  • Have been diagnosed with prediabetes

  • Have symptoms of diabetes. These include increased urination, increased thirst, increased hunger, and unexplained weight loss.

A diagnosis of diabetes

Any of the following test results confirm a diabetes diagnosis:

• A1C level of 6.5% or higher.

• Fasting plasma glucose (FPG) level of 126 mg/dL or higher. Fasting means not eating or drinking except water for at least 8 hours.

• Casual plasma glucose level of 200 mg/dL or higher, with the symptoms of high blood sugar (hyperglycemia) or hyperglycemic crisis. This level can be measured at any time of day.

• Oral glucose tolerance test level of 200 mg/dL or higher. For this test, you fast for at least 8 hours. Then you drink a sweet liquid that has a certain amount of glucose.

If you don't have symptoms of hyperglycemia, a diagnosis needs 2 abnormal test results from the same sample, or from 2 separate test samples. For example, an FPG of more than 126 and an A1C of more than 6.5% would be needed to diagnose diabetes.

Some conditions can affect the accuracy of the A1C test. These include:

• Sickle cell disease

• Pregnancy (second and third trimesters and the postpartum period)

• Glucose-6-phosphate dehydrogenase deficiency

• HIV

• Hemodialysis

• Recent blood loss or blood transfusion

• Erythropoietin therapy

A diagnosis of gestational diabetes

Gestational diabetes is diabetes that starts during pregnancy. All women who are at risk for type 2 diabetes should be tested for diabetes at their first prenatal visit. This is done using the standard tests above. More tests to check for gestational diabetes will be done later in pregnancy. Gestational diabetes may be diagnosed with 1 of the methods below.

One-step method

This is a 75-gram glucose tolerance test. For this test, you drink a sweet glucose drink. Blood sugar levels are checked after you have fasted for at least 8 hours. They are also checked 1 hour after the drink and 2 hours after the drink. This test is done at 24 to 28 weeks' gestation in women who have not been diagnosed with diabetes before. Gestational diabetes is diagnosed when you have any one of the following blood sugar levels: 

• Fasting: 92 mg/dL or higher

• 1 hour: 180 mg/dL or higher

• 2 hour: 153 mg/dL or higher

Two-step method

This test is also done at 24 to 28 weeks' gestation in women who have not been diagnosed with diabetes.

Step 1. A 50-gram glucose tolerance test with a blood sugar level check at 1 hour. If the blood sugar at 1 hour is 130 to 140 mg/dL or higher, then go to step 2.  

Step 2. A100-gram glucose tolerance test is done while you are fasting. Gestational diabetes is diagnosed when you have at least 2 of the following blood sugar levels:

• Fasting: 95 mg/dL or higher

• 1 hour: 180 mg/dL or higher

• 2 hour: 155 mg/dL or higher

• 3 hour: 140 mg/dL or higher

Talk with your healthcare provider about being screened for gestational diabetes. Women who have been diagnosed with gestational diabetes should be tested for diabetes at least every 3 years.