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Your doctor can diagnose diabetes or prediabetes using blood tests. The blood tests show if your blood glucose level, also called blood sugar, is higher than the range that is healthy for you. Blood tests can also help identify the type of diabetes you have.

Don’t try to diagnose yourself if you think you might have diabetes. Blood testing equipment that you can buy over the counter, such as a blood glucose meter, cannot diagnose diabetes.

What tests are used to diagnose diabetes?

  •       Fasting plasma glucose test

The fasting plasma glucose (FPG) test measures your blood glucose level at a single point in time. For the most reliable results, your doctor will give you the test in the morning after you have fasted for at least 8 hours. Fasting means having nothing to eat or drink except sips of water.

  •        A1C test

The A1C test is a blood test that provides your average levels of blood glucose over the last 3 months. Other names for the A1C test are hemoglobin A1C, HbA1C, glycated hemoglobin, and glycosylated hemoglobin test. You can eat and drink before this test.

Your doctor will report your A1C test result as a percentage, such as an A1C of 7%. The higher the percentage is, the higher your average blood glucose levels are.

  •     Random plasma glucose test

Sometimes doctors use the random plasma glucose test to diagnose diabetes when you have symptoms of diabetes and they do not want to wait until you have fasted for 8 hours. You may have this blood test at any time.

What test results tell me if I have diabetes or prediabetes?

Each test to detect diabetes and prediabetes uses a different measurement. Usually, your doctor will use a second test to confirm you have diabetes.

The table below helps you understand what your test results mean if you are not pregnant. If you are pregnant, some tests use different cutoffs. Ask your doctor what your test results mean.

Test results for diagnosis of prediabetes and diabetes

Diagnosis A1C Fasting Plasma Glucose Oral Glucose Tolerance Test* Random Plasma Glucose Test‡
Normal below 5.7% 99 mg/dL or below 139 mg/dL or below N/A
Prediabetes 5.7% to 6.4% 100 to 125 mg/dL 140 to 199 mg/dL N/A
Diabetes 6.5% or above 126 mg/dL or above 200 mg/dL or above 200 mg/dL or above

* 2 hours after glucose intake
‡ Used when there are symptoms
Source: American Diabetes Association