Hepatic Encephalopathy: Likelihood Ratios

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).

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The likelihood ratios of findings for Hepatic Encephalopathy are listed below. See the left navigation bar to change the display.

Good Negative Likelihood Ratio

Finding	+LR	-LR	Comments, Study
Arterial Ammonia	1.3	0.4	above upper limit of normal, 47 umol/L in patients with cirrhosis Study: Am J Med. 2003 Feb 15;114(3):188-93 PMID: 12637132

Poorly Diagnostic Findings

Finding	+LR	-LR	Comments, Study
Venous Ammonia	1.1	0.9	ammonia > upper limit of normal. in cirrhotics Note that in this study, all patients with acute liver failure had ammonia levels > upper limit of normal. However, ammonia levels did correlate with grade of HE (r=.91). Study: Clin Biochem. 2005 Aug;38(8):696-9 PMID 15963970

Findings With Unspecified Accuracy

Finding	+LR	-LR	Comments, Study
Asterixis	No accuracy specified.		Study: no study specified.
CSF Glutamine	No accuracy specified.		Per UpToDate, is sensitive but LP usually contraindicated due to coagulopathy. Study: no study specified.