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.