Measurement of the blood ammonia concentration is not always useful in patients with known or suspected hepatic encephalopathy. Ammonia contributes to hepatic encephalopathy; however, ammonia concentrations are much higher in the brain than in the blood and therefore do not correlate well. 28 Furthermore, ammonia is not the only waste product responsible for encephalopathy. Thus, blood ammonia concentrations show only a mediocre correlation with the level of mental status in patients with liver disease. It is not unusual for the blood ammonia concentration to be normal in a patient who is in a coma from hepatic encephalopathy.
Abnormal LFTs do not necessarily indicate any underlying abnormality of function. Traditionally, 'normal' values are defined as being within ± 2 standard deviations of the mean (of a normally distributed range), meaning that just over 95% of a healthy population will fall within the 'normal range', but % of a normal population will be normal outliers lying above it, and a further % will be normal outliers lying below it. However, as liver disease is frequently asymptomatic, this argument should not be used as an excuse for inadequate investigation. Abnormal LFTs are often inadequately investigated - which may miss an early opportunity of identifying and treating chronic liver disease [ 4 ] .