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 teststo 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.
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.
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.
This article was last reviewed on 2 April 2012. | This article was last modified on 17 September 2013.
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