Ammonia, Plasma

Ammonia is elevated in the following conditions: liver disease, urinary tract infection with distention and stasis, Reye syndrome, inborn errors of metabolism including deficiency of enzymes in the urea cycle, HHH syndrome (hyperammonemia-homocitrullinuria, hyperornithinemia), some normal neonates (usually returning to normal in 48 hours), total parenteral nutrition, ureterosigmoidostomy, and sodium valproate therapy. Ammonia determination is indicated in neonates with neurological deterioration, subjects with lethargy and/or emesis not explained, and in patients with possible encephalopathy.

Ammonia measurements are mainly of use in the diagnosis of urea cycle deficiencies (any neonate with unexplained nausea, vomiting, or neurological deterioration appearing after first feeding), and they play an important part in the detection of Reye syndrome.

In Reye syndrome threefold increases in AST, ALT and plasma ammonia are required for diagnosis with/or the diagnostic liver biopsy findings. Ammonia levels increase characteristically early; plasma ammonia ≥100 μg/dL reflects severe hepatic changes. Prothrombin time is increased in essentially all patients, prototypically three seconds longer than the control. Bilirubin is usually normal. Glucose should be monitored; hypoglycemia may develop. Hyperosmolality and acid-base imbalance may develop, lactate may increase, CK may increase and CK-MB may be elevated. Uric acid may increase.^1,2 Increased ammonia and prolonged prothrombin time provide indicators of disease progression.³