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.

To prevent the chance of damaging the artery of the wrist when the blood sample is taken, a procedure called the Allen test may be done to make sure that blood flow to your hand is normal. An ammonia test will not be done on an arm used for dialysis or if there is an infection or inflammation in the area of the puncture site.

The health professional taking a sample of your blood will:

• Clean the needle site with alcohol. You may be given an injection of local anesthetic to numb that area.
• Put the needle into the artery. More than one needle stick may be needed.
• Allow the blood to fill the syringe. Be sure to breathe normally while your blood is being collected.
• Put a gauze pad or cotton ball over the needle site as the needle is removed.
• Put a bandage over the puncture site and apply firm pressure for 5 to 10 minutes (possibly longer if you take blood-thinning medicine or have bleeding problems).

How It Feels

If the sample is taken from a vein

The blood sample is taken from a vein in your arm. An elastic band is wrapped around your upper arm. It may feel tight. You may feel nothing at all from the needle, or you may feel a quick sting or pinch.

If the sample is taken from an artery

Collecting blood from an artery is more painful than collecting it from a vein because the arteries are deeper and are protected by nerves.

• Most people feel a brief, sharp pain as the needle to collect the blood sample enters the artery. If you are given a local anesthetic, you may feel nothing at all from the needle puncture, or you may feel a brief sting or pinch as the needle goes through the skin.
• You may feel more pain if the person drawing your blood has a hard time finding your artery, your artery is narrowed, or if you are very sensitive to pain.

What Affects the Test

Reasons you may not be able to have the test or why the results may not be helpful include:

• Smoking.
• Eating a high-protein or a low-protein diet.
• Using medicines that increase blood ammonia levels, such as acetazolamide, valproate, and some diuretics (such as furosemide).
• Using medicines that decrease ammonia levels, such as diphenhydramine, isocarboxazid, lactulose, neomycin, phenelzine, tetracycline, and tranylcypromine.
• Strenuous exercise just before the test.

What To Think About

• Ammonia levels do not always reflect the severity of a person’s symptoms. For example, a person with severe cirrhosis may have only slightly elevated blood ammonia levels and yet may not be thinking clearly or may be sleepy or in a coma. Other people with very high ammonia levels may think and act normally.
• Symptoms of a high ammonia level, such as confusion or extreme sleepiness, may be treated with a medicine called lactulose, a laxative that works by reducing ammonia production in the intestines.
• It is common for newborns to have slightly high levels of ammonia in their blood. But the levels are temporary and usually do not cause symptoms.