What Tests Are Used to Diagnose Diabetes?

Tests for Type 1 and Type 2 Diabetes

  • Glycated hemoglobin (A1C) test: This blood test measures your average blood sugar level for the past two to three months. It measures the percentage of blood sugar attached to hemoglobin, which is the oxygen-carrying protein in red blood cells. The higher your blood sugar levels are, the more hemoglobin you’ll have with sugar attached. An A1C level of 6.5 percent or higher on two separate tests indicates that you have diabetes.

    If the two A1C test results aren’t similar, the test isn’t available, or if you have certain conditions that can make the A1C test inaccurate — such as if you’re pregnant or have an uncommon form of hemoglobin (known as a hemoglobin variant) — your doctor may also use the following tests to find out if you have diabetes:

    • Random blood sugar test: A blood sample will be taken randomly, regardless of when you last ate. A random blood sugar level of 200 mg/dL (11.1 mmol/L) or higher suggests that you may have diabetes.

    • Fasting blood sugar test: A blood sample will be taken after an overnight fast. A fasting blood sugar level between 100 and 125 mg/dL (5.6 and 6.9 mmol/L) is considered prediabetes. If it’s 126 mg/dL (7 mmol/L) or higher on two separate tests, you’ll be diagnosed with diabetes.

Other Tests

If your doctor thinks that you may have type 1 diabetes, your urine will be tested to look for the presence of ketones. Ketones are substances that are produced in your body when muscle and fat tissue are used for energy if your body doesn’t have enough insulin to use the available glucose.

Sometimes, a blood test called a C peptide level test, which measures the ability of your pancreas to secrete insulin, may be done as well. Levels of C peptide are usually low in people with type 1 diabetes, but may be normal or high in people with type 2 diabetes.

Guidelines and Tests for Gestational Diabetes

There is not a single set of screening guidelines for gestational diabetes. Therefore, your doctor will likely evaluate your risk factors for gestational diabetes early in your pregnancy.

  • If you’re at high risk of gestational diabetes, for example, your body mass index (BMI) before pregnancy was 30 or higher, you had gestational diabetes during a previous pregnancy or you have a mother, father, sibling or child with diabetes, your doctor may test for diabetes at your first pregnancy office visit.

  • If you’re at average risk of gestational diabetes, you’ll likely have a screening test for gestational diabetes sometime during your second trimester, typically between 24 and 28 weeks of pregnancy.

    Your doctor may use the following screening tests:

    • Initial glucose challenge test: You’ll begin the glucose challenge test by drinking a syrupy glucose solution. One hour later, you’ll have a blood test to measure your blood sugar level. A blood sugar level that is below 130 to 140 mg/dL or 7.2 to 7.8 mmol/L is usually considered normal on a glucose challenge test, although this may vary at specific clinics or labs. If your blood sugar level is higher than normal, it only means that you have a higher risk of gestational diabetes. Your doctor will order another test to determine if you have gestational diabetes.

    • Follow-up glucose tolerance testing: For this test, you’ll be asked to fast overnight and then have your fasting blood sugar level measured. Then you’ll drink another sweet solution, this one containing a higher concentration of glucose, and your blood sugar level will be checked every hour for a period of three hours. If at least two of the blood sugar readings are higher than normal, you’ll be diagnosed with gestational diabetes.

Tests for Prediabetes

  • The glycated hemoglobin (A1C) test: This blood test gives your average blood sugar level for the past two to three months. It works by measuring the percentage of blood sugar that is attached to hemoglobin (the oxygen-carrying protein in red blood cells). The higher your blood sugar levels, the more hemoglobin you’ll have with sugar attached. An A1C level between 5.7 and 6.4 percent suggests you have prediabetes while normal levels are below 5.7 percent.

    If the A1C test isn’t available or if you have certain conditions that can make the A1C test inaccurate — such as if you’re pregnant or have an uncommon form of hemoglobin (known as a hemoglobin variant) — your doctor may use the following tests to find out if you have diabetes:

  • Fasting blood sugar test: A blood sample will be taken after an overnight fast. A blood sugar level from 100 to 125 mg/dL (5.6 to 6.9 mmol/L) is considered prediabetes. Normal is below 100 mg/dL.

  • Oral glucose tolerance test: A blood sample will be taken after you fast for at least eight hours or overnight. Then you’ll drink a sugary solution, and your blood sugar level will be measured again after two hours. A blood sugar level less than 140 mg/dL (7.8 mmol/L) is normal. A blood sugar level from 140 to 199 mg/dL (7.8 to 11 mmol/L) is considered prediabetes.