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Diabetes Health Center

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Blood Glucose


Why It Is Done

Blood glucose tests are done to:

  • Check for prediabetes and diabetes.
  • Monitor treatment of diabetes.
  • Check for diabetes that occurs during pregnancy (gestational diabetes).
  • Determine if an abnormally low blood sugar level (hypoglycemia) is present. A test to measure blood levels of insulin and a protein called C-peptide may be done along with a blood glucose test to determine the cause of hypoglycemia. To learn more, see the topic C-Peptide.

How To Prepare

Fasting blood sugar (FBS)

This is one of several tests that is used to diagnose diabetes. For a fasting blood sugar test, do not eat or drink anything other than water for at least 8 hours before the blood sample is taken.

If you have diabetes, you may be asked to wait until you have had your blood tested before taking your morning dose of insulin or diabetes medicine. You may have a random blood sugar test instead, which will not require an 8-hour fast.

2-hour postprandial blood sugar

For a 2-hour postprandial test, start eating a meal exactly 2 hours before the blood sample is taken. A home blood sugar test is the most common way to check 2-hour postprandial blood sugar levels.

Random blood sugar (RBS) and hemoglobin A1c

No special preparation is required before having a random blood sugar or A1c test.

Oral glucose tolerance test

For an oral glucose tolerance test, you'll need to follow a special diet for 3 days before the test. And do not eat, drink, smoke, or exercise strenuously for at least 8 hours before your first blood sample is taken.

To learn more about how to prepare for this test, see Oral Glucose Tolerance Test.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean.

To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

The health professional taking a sample of your blood will:

  • Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
  • Clean the needle site with alcohol.
  • Put the needle into the vein. More than one needle stick may be needed.
  • Attach a tube to the needle to fill it with blood.
  • Remove the band from your arm when enough blood is collected.
  • Apply a gauze pad or cotton ball over the needle site as the needle is removed.
  • Apply pressure to the site and then a bandage.

WebMD Medical Reference from Healthwise

Last Updated: November 14, 2014
This information is not intended to replace the advice of a doctor. Healthwise disclaims any liability for the decisions you make based on this information.
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