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.
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. It is most commonly used to diagnose and monitor hepatic encephalopathy, a severe liver disease.
Ammonia (NH4+) is produced by cells throughout the body, especially the intestines, liver, and kidneys. 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 micrograms per deciliter (mcg/dL).
Note: Normal value ranges may vary slightly among different laboratories. Talk to your doctor about the meaning of your specific test results.
Conditions that can increase ammonia levels include:
There is very little risk involved with having your blood taken. Veins and arteries vary in size from one patient to another and from one side of the body to the other. Taking blood 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)
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.