Hepatic Encephalopathy: Sensitivity and Specificity
Introduction: See also Cirrhosis. As you can see below, measuring ammonia in known cirrhotics is of little use as it does not predict encephalopathy (though it does correlate modestly with the degree of encephalopathy). In acute liver failure, an ammonia > 200 mcg/dl predicts danger of herniation from cerebral edema (PMID 10051463).
There may be some utility in the serum ammonia in patients presenting with altered mental status without known cirrhosis, though there does not appear to be any data to this effect (for a discussion, see http://www.ccjm.org/content/76/4/252.full ). Note that hyperammonemia may be caused by liver failure as well as a number of other causes, including iatrogenic (valproate, salicylates, glycine, and TPN).