Digestive Disorders Health Center

An ammonia test measures the amount of ammonia in the blood. Most ammonia in the body forms when protein is broken down by bacteria in the intestines. The liver normally converts ammonia into urea, which is then eliminated in urine.

Ammonia levels in the blood rise when the liver is not able to convert ammonia to urea. This may be caused by cirrhosis or severe hepatitis.

For this test, a blood sample may be taken from either a vein or an artery.

Why It Is Done

An ammonia test is done to:

• Check how well the liver is working, especially when symptoms of confusion, excessive sleepiness, coma, or hand tremor are present.
• Check the success of treatment for severe liver disease, such as cirrhosis.
• Help identify a childhood disorder called Reye syndrome that can damage the liver and the brain. Ammonia testing can also help predict the outcome (prognosis) of a diagnosed case of Reye syndrome.
• Help predict the outcome (prognosis) of a diagnosed case of acute liver failure.
• Check the level of ammonia in a person receiving high-calorie intravenous (IV) nutrition (hyperalimentation).

How To Prepare

Do not eat, drink anything other than water, or smoke for 8 hours before having an ammonia blood test.

Avoid strenuous exercise just prior to having this test.

Tell your doctor if you:

• Are taking any medicines. Many medicines can interfere with test results. Your doctor may instruct you to stop taking certain medicines for several days before having an ammonia test.
• Smoke or drink alcohol.

Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results may mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).

How It Is Done

If the sample is taken from a vein

The health professional taking a sample of your blood will:

• Wrap an elastic band around your upper arm to stop the flow of blood. This makes the veins below the band larger so it is easier to put a needle into the vein.
• Clean the needle site with alcohol.
• Put the needle into the vein. More than one needle stick may be needed.
• Attach a tube to the needle to fill it with blood.
• Remove the band from your arm when enough blood is collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put pressure on the site and then put on a bandage.

If the sample is taken from an artery

A sample of blood from an artery is usually taken from the inside of the wrist (radial artery), but it can also be taken from an artery in the groin (femoral artery) or on the inside of the arm above the elbow crease (brachial artery). You will be seated with your arm extended and your wrist resting on a small pillow. The health professional drawing the blood may rotate your hand back and forth and feel for a pulse in your wrist.