The liver normally removes ammonia during urea synthesis.
Substantial liver failure must be present for blood ammonia to rise.
The test is primarily used to evaluate patients with mental status changes; elevated blood ammonia supports a diagnosis of hepatic encephalopathy.
Disrupted hepatic blood flow patterns in cirrhotic patients cause blood ammonia to elevate after milder degrees of liver failure in that group.
Arterial plasma is the best specimen; smoking should be avoided prior to the test (it raises blood ammonia).