A blood glucose test measures the amount of sugar (glucose) in a sample of your blood.

See also:

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

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.

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 6 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. It is also used to monitor patients who have the disease.

The carbohydrates you eat eventually end up as glucose in the blood. Glucose is a major source of energy for most cells of the body, including those in the brain.

Levels vary according to the laboratory, but in general up to 100 milligrams per deciliter (mg/dL) are considered normal.

Persons with levels between 100 and 126 mg/dL may have impaired fasting glucose or pre-diabetes. These levels are considered to be risk factors for type 2 diabetes and its complications.

Diabetes is typically diagnosed when fasting blood glucose levels are 126 mg/dL or higher.

Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.

Greater than normal levels (hyperglycemia) may indicate:

• Acromegaly (very rare)
• Cushing syndrome (rare)
• Diabetes mellitus
• Excessive food intake
• Impaired fasting glucose (prediabetes)
• Hyperthyroidism
• Pancreatic cancer
• Pancreatitis
• Pheochromocytoma (very rare)
• Too little insulin

Lower than normal levels (hypoglycemia) may indicate:

• Hypopituitarism
• Hypothyroidism
• Insufficient dietary intake
• Insulinoma (very rare)
• Too much insulin

Additional conditions under which this test may be performed:

• Acute adrenal crisis
• Cushing syndrome caused by adrenal tumor
• Cushing syndrome - exogenous
• Delirium
• Dementia
• Dementia due to metabolic causes
• Diabetic hyperglycemic hyperosmolar coma
• Diabetic ketoacidosis
• Diabetic nephropathy/sclerosis
• Ectopic Cushing syndrome
• Epilepsy
• Generalized tonic-clonic seizure
• Glucagonoma
• Islet of Langerhans' tumor
• Multiple endocrine neoplasia (MEN) I
• Pituitary Cushing's (Cushing's disease)
• Syphilis
• Syphilis - primary
• Syphilis - secondary
• Syphilis - tertiary
• Type 1 diabetes
• Type 2 diabetes
• Transient ischemic attack (TIA)

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 glucose levels.

Drugs that can increase glucose measurements include the following:

• Atypical antipsychotics, especially olanzapine
• Corticosteroids
• Diazoxide
• Intravenous dextrose
• Diuretics
• Epinephrine
• Estrogens
• Glucagon
• Isoniazid
• Lithium
• Phenothiazines
• Phenytoin
• Salicylates (acute toxicity -- see aspirin overdose)
• 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 -- 2008. Diabetes Care. 2008;31:S12-S54.