Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic. An elastic band is placed around the upper arm to apply pressure and cause the vein to swell with blood.

A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the band 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.

In infants or young children:

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

Fast for 8 - 12 hours. The health care provider may advise you to withhold drugs that may affect test results.

Drugs that can interfere with the test include thiazide or loop diuretics, barbiturates, acetazolamide, neomycin, and oral kanamycin. Consult the health care provider before this test if you are taking any of these medications.

When the needle is inserted to draw blood, you may feel moderate pain, or only a prick or stinging sensation. Afterward, there may be some throbbing.

This test may be performed when a condition that may cause toxic accumulation of ammonia is present or suspected.

Ammonia (NH4+) is produced by cells throughout the body, especially the intestines, liver, and kidneys. In the kidneys, ammonia plays a minor role in the acid/base balance, but is otherwise a metabolic waste product (primarily the result of protein metabolism).

Most of the ammonia produced in the body is used by the liver in the production of urea. Urea is also a waste product but is much less toxic than ammonia.

Ammonia is especially toxic to the brain and can cause confusion, lethargy, and sometimes coma.

The normal range is 15 - 45 mcg/dL (mcg/dL = micrograms per deciliter). Normal value ranges may vary slightly among different laboratories.

Conditions that can increase ammonia levels include:

• Liver failure
• Severe congestive heart failure
• Erythroblastosis fetalis
• Gastrointestinal (GI) bleeding - usually in the upper GI tract
• Genetic diseases of the urea cycle
• Leukemia
• Pericarditis
• Reye's syndrome
• Certain drugs

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

This test is about 90% accurate.

Patients with liver disease may have clotting problems. After the venipuncture, pressure should be applied to the puncture site for several minutes to ensure that bleeding has stopped.

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 may be more difficult from some people than from others.