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Also known as: NH3
Formal name: Ammonia, plasma
Related tests: Liver Panel, ALT, AST, ALP

At a Glance

Why Get Tested?

To help diagnose severe liver disease and certain genetic urea cycle disorders; to help investigate the cause of changes in behavior and consciousness; to support the diagnosis of hepatic encephalopathy and Reye’s syndrome

When to Get Tested?

When someone experiences mental changes or lapses into a coma of unknown origin; when an infant or child experiences frequent vomiting and increased lethargy as a newborn or about a week after a viral illness

Sample Required?

A blood sample drawn from a vein in your arm

Test Preparation Needed?

You should not smoke cigarettes prior to collection of the specimen.

The Test Sample

What is being tested?

This test measures the amount of ammonia in the blood. Ammonia is a compound produced by intestinal bacteria and by cells in the body during the digestion of protein. It is a waste product that is normally transported to the liver, where it is converted into urea and glutamine. The urea is then carried to the kidneys, where it is excreted in the urine. If this "urea cycle" does not complete the breakdown of ammonia, ammonia builds up in the blood and passes through the blood/brain barrier. Other sources of ammonia are kidney and muscle.

In the brain, ammonia and other compounds metabolized by the liver can accumulate and cause hepatic encephalopathy when liver function is reduced due to disorders such as cirrhosis or hepatitis. Hepatic encephalopathy causes mental and neurological changes that can lead to confusion, disorientation, sleepiness, and eventually to coma and even death. Infants and children with increased ammonia levels may vomit frequently, be irritable, and be increasingly lethargic. Left untreated, they may experience seizures, respiratory difficulty, and may lapse into a coma.

Problems with ammonia processing can arise from several sources, including:

  • Severe liver disease – damage limits the ability of the liver to metabolize ammonia; acute increases in ammonia may be seen in people with stable liver disease, especially following a triggering event such as gastrointestinal bleeding or an electrolyte imbalance.
  • Decreased blood flow to liver – ammonia is less able to get to the liver to be metabolized.
  • Reye's syndrome – a rare condition that affects the blood, brain, and liver; it is characterized by a rise in ammonia levels and a fall in glucose and affects primarily children and young adults. In most cases, it follows and appears to be triggered by a viral infection. Children who use aspirin are at an increased risk.
  • Renal failure – the kidneys are unable to effectively rid the body of urea, leading to a build-up of ammonia in the blood.
  • Rare inherited defects in the urea cycle – a deficiency or defect in one or more of the enzymes necessary to complete the conversion of ammonia to urea.

How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

Avoid smoking cigarettes prior to collection of the specimen and follow any other instructions you are given.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

Pagana K, Pagana T. Mosby's Manual of Diagnostic and Laboratory Tests, 4th edition. Pp 58-60.

ARUP Lab Tests. Ammonia, Plasma. Available online at through Accessed October 2011. 

MedlinePlus Medical Encylopedia. Essential hypertension. Available online at Accessed October 2011.

MedlinePlus Medical Encylopedia. Malignant hypertension. Available online at Accessed October 2011.

Elgouhari, HM. Q: What is the utility of measuring the serum ammonia level in patients with altered mental status? Cleveland Clinic Journal of Medicine. Available online at through Accessed October 2011.

Sources Used in Previous Reviews
Pagana, Kathleen D. & Pagana, Timothy J. (© 2007). Mosbys Diagnostic and Laboratory Test Reference 8th Edition: Mosby, Inc., Saint Louis, MO. Pp 52-53.

Clarke, W. and Dufour, D. R., Editors (2006). Contemporary Practice in Clinical Chemistry, AACC Press, Washington, DC. Pp 270.

Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 98-99.

Lehrer, J. (2006 October 13). Hepatic encephalopathy. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed on 2/17/08.

(2006 September, Updated). Recommended Newborn Screening Tests: 29 Disorders. March of Dimes [On-line information]. Available online through Accessed on 12/17/08.

Alerif, L. (2008 January 22, Updated). Hereditary urea cycle abnormality. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed on 2/17/08.

Van Voorhees, B. (2006 August 18). Reye syndrome. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed on 2/17/08.

Hepatocerebral degeneration. MedlinePlus Medical Encyclopedia [On-line information]. Available online at Accessed on 2/17/08.

(© 2005). What is a Urea Cycle Disorder? National Urea Cycle Disorders Foundation [On-line information]. Available online through Accessed on 2/17/08.

KidsHealth, Nemours Foundation. InfectionsReye Syndrome (online information). Available online through Accessed March 2008.

S. Bakerman (2002). Bakermans ABCs of Interpretive Laboratory Data (4th ed), Interpretive Laboratory Data, Scottsdale, AZ, Pp. 44-45.

Thomas, Clayton L., Editor (1997). Tabers Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

(© 2004) Ammonia. ARUP's Guide to Clinical Laboratory Testing [On-line information]. Available online at through

(2004, May 24, Updated). Medical Management Guidelines (MMGs) for Ammonia (NH3). Agency for Toxic Substances and Disease Registry [On-line information]. Available online through

Miller, K. (2003 October 1). Do Ammonia Levels Correlate with Hepatic Encephalopathy? American Family Physician, Tips from Other Journals [On-line journal]. Available online through

Johnston, D. (1999 April 15). Special Considerations in Interpreting Liver Function Tests. American Family Physician [On-line journal]. Available online through

(© 1995-2004). Portal-Systemic Encephalopathy (Hepatic Encephalopathy; Hepatic Coma). The Merck Manual of Diagnosis and Therapy, Section 4. Hepatic And Biliary Disorders, Chapter 38. Clinical Features Of Liver Disease [On-line information]. Available online through

(© 1995-2004). Liver Encephalopathy. The Merck Manual Second Home Edition, Section 10. Liver and Gallbladder Disorders, Chapter 135. Clinical Manifestations of Liver Disease [On-line information]. Available online through

(© 1998-2003). Understanding the Basics, What is a Urea Cycle Disorder? National Urea Cycle Disorders Foundation [On-line information]. Available online through

Burnett, R. (2002 September). September 2002 Q & A. College of American Pathologists [On-line information]. Available online through

Mayo Clinic staff (2004 February 18). Reye's syndrome. [On-line information]. Available online through

National Academy of Clinical Biochemistry. Laboratory Guidelines for Screening, Diagnosis and Monitoring of Hepatic Injury (2000). Available online through