The ammonia test is used to detect an elevated level in the blood that can be caused by severe liver disease, kidney failure, Reye syndrome or a rare genetic disorder of the urea cycle. It may be used to help investigate the cause of an individual's changes in behavior and consciousness. This test may be used to diagnose the cause of a coma or to help support the diagnosis of Reye syndrome or hepatic encephalopathy caused by various liver diseases. The test can also be used to monitor the effectiveness of treatment of hepatic encephalopathy.