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. Carbohydrates (or carbs) are found in fruit, cereal, bread, pasta, and rice. They are quickly turned into glucose in your body. This raises your blood glucose level.

See also:

• Glucose test - CSF
• Glucose test - urine
• Glucose tolerance test
• Home blood glucose monitoring

Random blood sugar; Blood sugar level; Fasting blood sugar

A blood sample is needed. For information on how this is done, see: Venipuncture.

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.

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.

Your doctor may order this test if you have signs of diabetes. However, other tests (glucose tolerance test and fasting blood glucose test) are better for diagnosing diabetes.

The blood glucose test is also used to monitor patients who have the diabetes. It may also be done if you have:

• A change in behavior
• Fainting spells
• Seizures for the first time

Up to 100 milligrams per deciliter (mg/dL) is considered normal for a fasting blood glucose test.

Persons with levels between 100 and 125 mg/dL have impaired fasting glucose, a type of prediabetes. These levels are considered 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.

The examples above show the common measurements for results for these tests. Some laboratories use different measurements or may test different specimens.

Higher-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:

• Impaired fasting glucose (also called "prediabetes")
• Overactive thyroid gland
• Pancreatic cancer
• Pancreatitis
• Pheochromocytoma,acromegaly,Cushing syndrome, or glucagonoma (all of which are rare causes)

Lower-than-normal levels (hypoglycemia) may be due to:

• Hypopituitarism
• Underactive thyroid gland
• Insulinoma (very rare)
• Too little food
• Too much insulin or other diabetes medications

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 raise blood glucose levels.

Drugs that can increase glucose measurements include the following:

• Atypical antipsychotic medications, especially olanzapine, quetiapine, and risperidone
• Beta-blockers (such as propranolol)
• Corticosteroids
• Dextrose
• Epinephrine
• Estrogens
• Glucagon
• Isoniazid
• Lithium
• Oral contraceptives (birth control pills)
• Phenothiazines
• Phenytoin
• Salicylates (see aspirin overdose)
• Thiazide diuretics
• Triamterene
• Tricyclic antidepressants

Drugs that can decrease glucose measurements include the following:

• Acetaminophen
• Alcohol
• Anabolic steroids
• Clofibrate
• Disopyramide
• Gemfibrozil
• Monoamine oxidase inhibitors (MAOIs)
• Pentamidine
• Sulfonylurea medications (such as glipizide, glyburide, and glimepiride)

American Diabetes Association. Standards of medical care in diabetes -- 2011. Diabetes Care. 2011;32:S11-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.