Fasting Blood Glucose Test

A primer on what it's used for and how it's done

The fasting blood glucose test is the test most commonly used to diagnose diabetes. It measures blood glucose levels after a period of fasting, usually at least eight hours without food or liquid (except water). This test is more definitive than a random test, because there is no chance that it has been influenced by recent food intake.

If your fasting blood glucose level is greater than 125 mg/dL in two separate tests, then the diagnosis of diabetes is made.

What it's used for

This test is used to measure your level of fasting blood glucose to diagnose diabetes.

Preparation

Don't eat or drink anything but water for eight hours before the test is scheduled. Most of the time, this means having the test done in the morning, before breakfast.

How it's done

A small amount of blood is collected either from a finger or from a vein. The blood is then analyzed and you will be informed of the results.

Follow-up

If the first level is above 125 mg/dL, the fasting blood glucose test will be done again (for example, the next day) to confirm a diagnosis of diabetes. A value between 100 and 125 mg/dL indicates pre-diabetes. Someone with a result in this range should be followed closely, with follow-up testing.

Risks

There are no risks to this test.

Call your doctor if you have any questions about what the test means.

Additional information

Fasting blood glucose less than 100 mg/dL is normal.

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