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Fasting Blood Glucose

Alternate Names

  • FBS
  • fasting blood sugar

Definition

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.

Who is a candidate for the test?

If a doctor suspects a person may have diabetes or pre-diabetes due to certain symptoms or risk factors, this test may be done.

How is the test performed?

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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.

What is involved in preparation for the test?

A person should have nothing to eat or drink other than water for at least 8 hours before this test.

What do the test results mean?

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.

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