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
A blood sample taken from a vein or artery in your arm
Avoid smoking cigarettes prior to collection of the specimen and follow any other instructions that you are given
This test measures the concentration of ammonia in the blood. Ammonia is a chemical produced by bacteria in the intestine and by cells in the body during the processing of proteins. Ammonia is a poisonous waste product which is normally transported to the liver, and then converted into two chemicals called urea and glutamine. The urea is then carried by the blood to the kidneys, where it is excreted in the urine. If this does not work correctly, ammonia can build up in the blood and pass into the brain.
Accumulation of ammonia and other compounds normally broken down by the liver can cause a condition affecting the brain called hepatic encephalopathy. This causes symptoms such as confusion, disorientation, drowsiness, and eventually coma and even death. Infants and children with increased blood ammonia concentrations may vomit frequently, be irritable, and be increasingly tired. If left untreated, they may develop seizures, breathing difficulty, and fall into a coma.
Ammonia can accumulate in the bloodstream due to several reasons, including:
- Rare inherited defects in the urea cycle - a deficiency or inborn genetic defect affecting one or more of the enzymes necessary to complete the conversion of ammonia to urea.
- Severe liver disease - damage which reduces the ability of the liver to breakdown and remove ammonia. Sudden increases in blood ammonia concentration may be seen in patients with stable liver disease, especially following an event such as a bleed into the stomach or elsewhere in the intestine or an imbalance in sodium or other electrolytes.
- Decreased blood flow to the liver such that ammonia is less able to be removed.
- Reye’s syndrome - a rapidly developing condition that affects the blood, brain, and liver. It typically produces a rise in ammonia 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.
- Kidney failure - the kidneys are unable to effectively remove the body’s urea, leading to a build-up of ammonia in the blood.
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?
Samples must be sent to the laboratory prompty for analysis.
How is it used?
The ammonia test can be used to help investigate the cause of changes in behaviour and consciousness. It may be requested with other tests such as glucose, electrolytes, and kidney and liver function tests to help diagnose the cause of a coma or to help support the diagnosis of Reye’s syndrome or hepatic encephalopathy. An ammonia blood test may also be requested to help detect and evaluate the severity of a urea cycle defect.
Some doctors use the ammonia test to monitor the effectiveness of treatment of hepatic (liver) encephalopathy, but there is not widespread agreement on how best to use the test clinically. Since hepatic encephalopathy can be caused by the build-up of a variety of other poisonous substances in the blood and brain, blood ammonia concentrations may not be very good at showing how bad the disease is.
When is it required?
An ammonia blood test may be requested on a newborn when they are irritable, vomiting, tired, and have seizures during the first few days after birth. It may be performed when a child develops these symptoms about a week after a viral illness such as flu or a cold when the doctor suspects that the child may have Reye’s syndrome.
When adults experience mental changes, disorientation, sleepiness, or lapse into a coma, an ammonia blood test may be requested to help investigate the cause of the change in consciousness. In patients with stable liver disease, an ammonia blood test may be requested, with other liver function tests, when a patient suddenly “takes a turn for the worse” and becomes more acutely ill.
What does the test result mean?
Significantly increased concentrations of ammonia in the blood indicate that the body is not effectively breaking down and removing ammonia but do not explain the cause. In infants, extremely high concentrations are can be seen with an inherited urea cycle enzyme deficiency or defect but may also be seen with haemolytic disease of the newborn (where the baby’s red blood cells are destroyed). Short-lived increases in blood ammonia are relatively common in newborns, where levels may rise and fall without causing detectable symptoms.
Increased ammonia levels and decreased glucose levels may suggest the presence of Reye’s syndrome in children and adolescents with symptoms. Increased ammonia concentrations in the blood may also suggest a previously undiagnosed enzymatic defect of the urea cycle. In children and adults, elevated ammonia levels may also indicate liver or kidney damage. Frequently, an illness will act as a trigger, increasing ammonia levels to the point that an affected patient has difficulty removing the ammonia.
Normal concentrations of ammonia do not rule out the possibility of hepatic (liver) encephalopathy. Not only do other waste products contribute to the changes in mental function and consciousness, but brain levels of ammonia may be much higher than blood levels. In this situation, patients with severe symptoms do not always have very high blood ammonia levels.
Is there anything else I should know?
Increased concentrations of ammonia may also be seen with:
- Gastrointestinal bleeding – blood cells are haemolysed (broken apart) in the intestines, releasing protein which is digested, absorbed and converted into ammonia.
- Muscle exercise – muscles produce ammonia when active and absorb it when resting.
- Tourniquet use – ammonia can be increased in blood samples collected using a tourniquet when it has been applied for a long time.
- Drugs that can increase blood ammonia concentration include: alcohol, barbiturates, diuretics, valproic acid and narcotics
Decreased concentration of ammonia may be seen with high blood pressure and the use of some antibiotics (such as neomycin).
Ammonia tests can also be measured on blood taken from arteries, but this is rarely done although some doctors believe that arterial ammonia measurements are more useful.
Is ammonia testing used to detect or monitor ammonia poisoning?
No, it is not useful for this purpose. In most cases, ammonia acts locally, burning or irritating whatever it comes in contact with but, it does not usually act as a poison to the whole body. Concentrated commercial ammonia, in a liquid or vapour form, causes more severe burns than the more dilute liquid household ammonia, but both can cause damage to the eyes, skin, respiratory tract, and to the mouth, throat, and stomach if swallowed.
When increased ammonia levels have returned to normal, can they rise again?
It depends on why they were increased in the first place. If the condition was temporary it is likely that the ammonia levels will continue to be normal. If the condition is long lasting (chronic) it is possible that they will increase again and health status will likely need to be monitored.
Elsewhere On The Web
Analyte Monographs alongside the National Laboratory Medicine Catalogue (AMALC): Ammonia
NHS Choices: Reye's Syndrome
National Urea Cycle Disorders Foundation
National Reye's Syndrome Foundation
NINDS Encephalopathy Information Page
MedlinePlus Medical Encyclopedia: Hepatic encephalopathy