Although the exact pathophysiology of hepatic encephalopathy (HE) is not fully understood, for more than a century ammonia has been thought to play a critical role. However, the interpretation and utility of ammonia levels in patients with chronic liver disease (CLD) presenting with HE has been a long-standing source of confusion. It is a common belief in the emergency department and on the wards that a single elevated ammonia level in a patient with CLD can confirm the diagnosis of HE, and a normal level essentially rules it out. This confusion stems from the fact that early studies showed a correlation between degree of encephalopathy and the ammonia level, but numerous subsequent studies have shown that severely encephalopathic patients often have normal ammonia levels. This paper reviews the published literature on ammonia levels in patients with CLD in an attempt to clarify its value as a clinical decision-making tool in patients with suspected HE.
