This test measures the amount of glucose in the blood. It is called fasting because the level is measured at least 8 hours after a person last ate or drank anything.
If a doctor suspects a person may have diabetes or pre-diabetes due to certain symptoms or risk factors, this test may be done.
In order to measure the amount of glucose in the blood, a blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a rubber tube called a tourniquet is tied around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle and is collected in a syringe or vial for testing in the laboratory. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
A person should have nothing to eat or drink other than water for at least 8 hours before this test.
The fasting blood glucose in a person without diabetes should be 99 mg/dL or less. If the result is higher than 99 mg/dL but less than 126 mg/dl, the diagnosis of impaired fasting glucose or pre-diabetes can be made.If the result is 126 mg/dL or higher, then another test should be done to confirm a diagnosis of diabetes.