Ammonia Measurement in Hepatic Encephalopathy

Initial Diagnostic Role

A normal ammonia level should prompt a reevaluation of the diagnosis of hepatic encephalopathy and a search for alternative causes of altered mental status in patients with suspected hepatic encephalopathy 1, 2, 3, 4
The American Association for the Study of Liver Diseases (AASLD) and European Association for the Study of the Liver (EASL) guidelines state that increased blood ammonia alone does not add diagnostic, staging, or prognostic value for hepatic encephalopathy in patients with chronic liver disease 1, 2, 3, 4, 5

Hepatic encephalopathy is diagnosed and graded by clinical criteria using the West Haven Criteria and Glasgow Coma Scale, not by ammonia levels 1, 2, 3, 4, 5

For ammonia-lowering drugs, repeated measurements of ammonia may be helpful to test drug efficacy in research or pharmacologic evaluation contexts 1, 2, 3, 5

Ammonia levels should be collected from fasting patients when possible, avoiding venous stasis, and using EDTA-containing tubes, placed immediately on ice, and transported to the laboratory within 60-90 minutes 6, 7
Sample hemolysis, high lipemia, jaundice, high-protein diet, smoking, and exercise can interfere with ammonia level results 6, 7

Ammonia levels should not be used to determine hepatic encephalopathy severity or grade, as this is done clinically 1, 2, 3, 4, 5
Elevated ammonia levels do not confirm hepatic encephalopathy, as ammonia may be elevated in cirrhotic patients without encephalopathy 6, 7
A normal ammonia level should prompt an investigation into alternative diagnoses, including infections, metabolic derangements, intracranial hemorrhage, drug effects, and alcohol withdrawal 8