Ammonia
The Test
How is it used?
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 ordered?
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. 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?
Increased ammonia levels and decreased glucose levels may indicate the presence of Reyes 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. Other wastes can contribute to changes in mental function and consciousness, and brain levels of ammonia may be much higher than blood levels. This can make correlation of patient symptoms to ammonia blood levels difficult.
Is there anything else I should know?
Decreased levels of ammonia may be seen with hypertension and the use of some antibiotics, such as neomycin.
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, but there is not widespread agreement on this.
