Plasma Ammonia

This test measures plasma levels of ammonia, a nonprotein nitrogen compound that helps maintain acid-base balance. In diseases such as cirrhosis of the liver, ammonia can bypass the liver and accumulate in the blood. Plasma ammonia levels may help indicate the severity of hepatocellular damage.

Purpose

• To help monitor the progression of severe hepatic disease and the effectiveness of therapy
• To recognize impending or established hepatic coma

Patient Preparation

• Explain to the patient (or to a family member if the patient is comatose) that this test is used to evaluate liver function.
• Tell him that the test requires a blood sample. Explain who will perform the venipuncture and when.
• Inform him that he may experience transient discomfort from the needle puncture and the tourniquet, but reassure him that collecting the sample takes only a few minutes.
• Check the patient's history for drugs that influence plasma ammonia levels. They include acetazolamide, thiazides, ammonium salts, furosemide, lactulose, neomycin, and kanamycin.

Procedure And Posttest Care

• Perform a venipuncture, and collect the sample in a 10-ml green-marble­top (heparinized) tube.
• Make sure bleeding has stopped before removing pressure from the venipuncture site.
• If a hematoma develops at the puncture site, apply warm soaks.
• Watch for signs of impending or established hepatic coma if plasma ammonia levels are high.

Precautions

• Notify the laboratory before performing the venipuncture so that preliminary preparations can begin.
• Handle the sample gently to prevent hemolysis, pack it in ice, and send it to the laboratory immediately.
• Do not use a chilled container.

Reference values

Normally, plasma ammonia levels in adults range from 9 to 33 mol/L.

Abnormal Findings

Elevated plasma ammonia levels are common in severe hepatic disease, such as cirrhosis and acute hepatic necrosis, and can lead to hepatic coma. Elevated levels may also occur in Reye's syndrome, severe heart failure, GI hemorrhage, and erythroblastosis fetalis.

Interfering Factors

• Hemolysis due to rough handling of the sample
• Delay in testing
• Acetazolamide, thiazides, ammonium salts, and furosemide (increase)
• Parenteral nutrition or a portacaval shunt (possible increase)
• Lactulose, neomycin, and kanamycin (decrease)
• Smoking, poor venipuncture technique, and exposure to ammonia cleaners in the laboratory (possible increase)