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.

Hormones made in the body called insulin and glucagon help control blood glucose levels.

See also:

• Glucose tolerance test

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 in 2 ways:

• After you have not eaten anything for at least 8 hours (fasting)
• At any time of the day (random)

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

If you had a fasting blood glucose test, a level between 70 and100 milligrams per deciliter (mg/dL) is considered normal.

If you had a random blood glucose test, normal results depend on when you last ate. Most of the time, blood glucose levels will be below 125 mg/dL.

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.

If you had a fasting blood glucose test:

• A level of 100-125mg/dL means you have impaired fasting glucose, a type of prediabetes. This increases your risk for type 2 diabetes.
• A level of 126 mg/dL and higher most often means you have diabetes.

Higher-than-normal random blood glucose levels may be a sign of diabetes. In someone with diabetes, it may mean the diabetes is not well controlled.  Your healthcare provider will likely order a fasting blood glucose or a glucose tolerance test, depending on your random test result.

Other medical problems may also cause higher-than-normal blood glucose levels, including:

• Overactive thyroid gland
• Pancreatic cancer
• Pancreatitis
• Rare tumors, including pheochromocytoma,acromegaly,Cushing syndrome, or glucagonoma

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

• Hypopituitarism (a pituitary gland disorder)
• 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:

• Certain medicines to treat schizophrenia and psychosis
• Beta-blockers (such as propranolol)
• Corticosteroids (such as prednisone)
• Estrogens
• Glucagon
• Isoniazid
• Lithium
• Oral contraceptives (birth control pills)
• Phenothiazines
• Phenytoin
• Salicylates (see aspirin overdose)
• Thiazide diuretics (such as hydrochlorothiazide)
• Triamterene
• Tricyclic antidepressants

Drugs that can decrease glucose measurements include the following:

• Acetaminophen
• Alcohol
• Anabolic steroids
• Clofibrate
• Disopyramide
• Gemfibrozil
• Monoamine oxidase inhibitors (MAOIs)
• Pentamidine

American Diabetes Association. Standards of medical care in diabetes--2012. Diabetes Care. 2011 Jan;35 Suppl 1:S11-63.

Buse JB, Polonsky KS,Burant CF. Type 2 diabetes mellitus. In: Melmed S, Polonsky KS,Larsen PR, Kronenberg HM, Larsen PR, eds.Williams Textbook of Endocrinology. 12th ed. Philadelphia,Pa: Saunders Elsevier; 2011:chap 31.

Inzucchi SE, Sherwin RS. Type 2 diabetes mellitus. In: Goldman L, Schafer AI, eds. Cecil Medicine. 24th ed. Philadelphia, PA:Saunders Elsevier; 2011:chap 237.