At a Glance
Why Get Tested?
To detect elevated levels of ammonia in the blood, to evaluate changes in consciousness, or to help diagnose hepatic encephalopathy and inborn errors of metabolism e.g. errors in the synthesis of urea.
When to Get Tested?
Venous is arterial ammonia is most often measured in the intensive care unit when acute liver failure is suspected. If a patient experiences mental changes or lapses into a coma of unknown origin; if an infant or child experiences confusion, frequent vomiting and increased lethargy as a newborn or about a week after a viral illness
A blood sample drawn from a vein or artery in your arm
Test Preparation Needed?
The Test Sample
What is being tested?
This test measures the amount of ammonia in the blood. Most of the ammonia is produced by intestinal bacteria and by cells in the body during the digestion of protein. Skeletal muscle produces some ammonia and this may be increased by seizures or intensive exercise. A waste product, ammonia is normally transported to the liver, where it is converted into urea. The urea is then carried by the blood to the kidneys, where it is excreted in the urine. If ammonia is not efficiently converted to urea by the liver it builds up in the blood and passes through the blood/brain barrier.
In the brain, ammonia and other compounds metabolised by the liver can cause hepatic encephalopathy – 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:
- 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
- Severe liver disease – damage limits the ability of the liver to metabolise ammonia. Acute increases in ammonia may be seen in patients with stable liver disease, especially following a triggering event such as gastrointestinal bleeding or an electrolyte imbalance
- In liver disease blood from the intestine carrying ammonia may be shunted into the general circulation without passing through the liver and being converted to urea.
- Drugs – some drugs e.g. the antiepileptic Valproate and even salicylate may sometimes lead to increased ammonia. Salicylate is thought to be involved in Reye syndrome, a cause of increased ammonia in children.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein or artery in the arm.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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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.