Why get tested
To detect elevated levels of ammonia in the blood, to evaluate changes in consciousness, or to help diagnose hepatic encephalopathy and Reye’s syndrome
When to get tested
If a patient experiences mental changes or lapses into a coma of unknown origin; if an infant or child experiences frequent vomiting and increased lethargy as a newborn or about a week after a viral illness
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.
A waste product,
ammonia is normally transported to the liver where it is converted into urea and glutamine.
The urea is then carried by the blood to the kidneys,
where it is excreted in the urine. If this “urea cycle” does not complete, ammonia builds up in the blood and passes through the blood/brain barrier.
In the brain, ammonia and other compounds metabolized 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 metabolize 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.
• Decreased blood flow to liver – ammonia is less able to get to the liver.
• Reye’s syndrome – an acute condition that affects the blood, brain, and liver. It is characterized by a rise in ammonia levels and a fall in glucose and affects 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.
A blood sample drawn from a vein or artery in your arm
specimen should be placed immediately on ice