How is it used?
The commonest cause of raised ammonia levels in the blood is acute liver failure where the levels are predicitive of severe cerebral complications. The ammonia test is primarily used to help investigate the cause of changes in behaviour and consciousness. It may be ordered, along with other tests (such as glucose, electrolytes, and kidney and liver function tests), to help diagnose the cause of a coma of unknown origin or to help support the diagnosis of Reye’s syndrome or hepatic encephalopathy. An ammonia level may also be ordered 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 encephalopathy, but there is not widespread agreement on its clinical utility. Since hepatic encephalopathy can be caused by the build-up of a variety of toxins in the blood and brain, blood ammonia levels correlate poorly with the degree of impairment.
When is it requested?
An ammonia test may be ordered on a newborn when symptoms such as irritability, vomiting, lethargy and seizures arise in the first few days after birth. It may be performed when a child develops these about a week following a illness such as influenza or a cold and 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 level may be ordered to help evaluate the cause of the change in consciousness especially with pre-existing liver disease. In patients with stable liver disease, an ammonia level may be ordered, along 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 metabolising and eliminating ammonia but do not indicate the cause. In infants, extremely high levels are associated with an inherited urea cycle enzyme deficiency or defect. Moderate short-lived increases in ammonia are relatively common in newborns, where the levels may rise and fall without causing detectible symptoms. Acute illness and seizures may cause raised blood ammonia levels, especially in children.
Increased ammonia levels and decreased glucose levels may indicate the presence of Reye’s syndrome in symptomatic children and adolescents. Increased concentrations may also indicate 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 acute or chronic illness will act as a trigger, increasing ammonia levels to the point that an affected patient has difficulty clearing them.
Normal concentrations of ammonia do not rule out hepatic encephalopathy. Not only do other wastes contribute to the changes in mental function and consciousness, but brain levels of ammonia may be much higher than blood levels, making correlation of symptoms to blood levels of ammonia difficult.
Is there anything else I should know?
Increased levels of ammonia may also be seen with:
- Gastrointestinal bleeding – blood cells are (broken apart) in the intestines, releasing protein.
- Muscular exertion – muscles produce ammonia when active and absorb it when resting.
- Tourniquet use – ammonia levels can be increased in the blood sample collected.
- Drugs that can increase ammonia include: valproate and salicylate
Ammonia tests can also be performed on arterial blood, but this method is much less frequently used. Some doctors feel that arterial ammonia measurements are more clinically useful and guidelines suggest that they are helpful for recognising hepatic encephalopathy, but there is not widespread agreement on this.