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

Blood is drawn from a vein (venipuncture), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood).

A needle is inserted into the vein, and the blood is collected in an airtight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding.

Infant or young child:

The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

You should not eat for 6 hours before the test. A random serum 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 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.

This test is used to evaluate blood glucose levels. It may be used to diagnose or screen for diabetes and to monitor control in patients who have diabetes.

Most dietary carbohydrates 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 up to 100 mg/dL are considered normal.

Levels between 100 and 126 mg/dl are referred to as 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: mg/dL = milligrams per deciliter

Greater than normal levels (hyperglycemia) may indicate:

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

Lower than normal levels (hypoglycemia) may indicate:

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

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)

• Excessive bleeding
• Fainting or feeling lightheaded
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

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)

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.