Ammonia, Plasma

The Ammonia, Plasma test (also known as plasma ammonia or ammonia [plasma]) is used to evaluate suspected hyperammonemia and to assess hepatic dysfunction. It supports the evaluation of possible hepatic encephalopathy and assists in the diagnostic workup for inborn errors of metabolism affecting the urea cycle as well as in the detection of Reye syndrome. Testing is indicated in neonates who develop neurologic deterioration or unexplained lethargy and/or vomiting, particularly following initial feedings. In appropriate clinical settings, rising ammonia in conjunction with a prolonged prothrombin time may serve as a marker of disease progression.

Blood ammonia concentration shows poor correlation with the severity of hepatic coma, and a single result does not reliably predict impending hepatic coma. Furthermore, not all individuals with urea cycle disorders exhibit elevated ammonia concentrations.