A blood glucose test measures the amount of a sugar, called glucose, in a sample of your blood.
Glucose is a major source of energy for most cells of the body, including those in the brain. The carbohydrates you eat eventually end up as glucose in the blood.
Random blood sugar; Blood sugar level; Fasting blood sugar
How the test is performed
Blood is typically drawn from a vein, usually from the inside of the elbow or the back of the hand. The site is cleaned with germ-killing medicine (antiseptic). The health care provider wraps an elastic band around the upper arm to apply pressure to the area and make the vein swell with blood.
Next, the health care provider gently inserts a needle into the vein. The blood collects into an airtight vial or tube attached to the needle. The elastic band is removed from your arm.
Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.
In infants or young children, a sharp tool called a lancet may be used to puncture the skin and make it bleed. The blood collects into a small glass tube called a pipette, or onto a slide or test strip. A bandage may be placed over the area if there is any bleeding.
How to prepare for the test
The test may be done while you are fasting or at random.
If you are having a fasting glucose blood test, you should NOT eat or drink for 8 hours before the test.
A random glucose test can be done at any time of the day, but results depend on what you drink or eat before the test, as well as your activity.
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
Your doctor may order this test if you have signs of diabetes. However, other tests (glucose tolerance test and fasting blood glucose test) must be used to diagnose diabetes.
The blood glucose test is also used to monitor patients who have the diabetes. It may also be done if you have:
- A seizure for the first time
- Strange changes in behavior
- Fainting spells
Levels vary according to the laboratory, but in general up to 100 milligrams per deciliter (mg/dL) are considered normal for a fasting blood glucose test.
Persons with levels between 100 and 125 mg/dL have impaired fasting glucose, or prediabetes. These levels are considered to be risk factors for type 2 diabetes and its complications.
Diabetes is diagnosed in persons with fasting blood glucose levels that are 126 mg/dL or higher.
Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
What abnormal results mean
Greater-than-normal blood glucose levels (hyperglycemia) may be a sign of diabetes. In someone with diabetes, it may mean the diabetes is not well controlled.
Increased levels may also be due to:
Lower-than-normal levels (hypoglycemia) may indicate:
What the risks are
Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.
Other risks associated with having blood drawn are slight but may include:
- Excessive bleeding
- Fainting or feeling light-headed
- Hematoma (blood accumulating under the skin)
- Infection (a slight risk any time the skin is broken)
Many forms of severe stress (for example, trauma, stroke, heart attack, and surgery) can temporarily increase blood glucose levels.
Drugs that can increase glucose measurements include the following:
- Atypical antipsychotics, especially olanzapine and risperidone
- Salicylates (see aspirin overdose)
- Tricyclic antidepressants
Drugs that can decrease glucose measurements include the following:
- Anabolic steroids
- Monoamine oxidase inhibitors (MAOIs)
- Sulfonylurea medications (such as glipizide, glyburide, and glimepiride)
American Diabetes Association. Standards of medical care in diabetes -- 2009. Diabetes Care. 2009;32:S13-S61.
Inzucchi SE, Sherwin RS. Type 2 diabetes mellitus. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 248.
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Review Date: 5/23/2010
Review By: David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine. Also reviewed by David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.
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