Ammonia Levels and Hepatic Encephalopathy in Patients with Known Chronic Liver Disease

© 2017 Society of Hospital Medicine

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Ammonia is predominantly generated in the gut by intestinal bacteria and enzymes and detoxified primarily in the liver. Since the 1930s, ammonia has been identified as the principal culprit in hepatic encephalopathy (HE). Many physicians utilize serum ammonia to diagnose, assess severity, and determine the resolution of HE in patients with chronic liver disease (CLD) despite research showing that ammonia levels are unhelpful in all of these clinical circumstances. HE in patients with CLD is a clinical diagnosis of exclusion that should not be based on ammonia levels.

A 62-year-old man diagnosed with cirrhosis due to Hepatitis C and alcoholism was brought to the emergency department for alteration in mentation. He had scant melenic stools 5 days preceding his admission and did not exhibit overt signs or symptoms of infection. His systemic examination was normal except for somnolence, disorientation to space and time, asterixis, and ascites. His lab parameters were within normal limits except for an elevated blood urea nitrogen and thrombocytopenia. His blood cultures did not grow any organisms, and paracentesis ruled out spontaneous bacterial peritonitis. During his hospital stay, he underwent esophageal variceal banding and was effectively managed with lactulose and rifaximin. The patient was alert, fully oriented, and without asterixis at the time of discharge 6 days later. Would an elevated venous ammonia level at admission alter management? If the ammonia level was elevated, would serial ammonia measurements affect management?

The colonic microbiome produces ammonia from dietary nitrogen. In health, approximately 85% of it is detoxified by the liver and excreted as urea in urine, while muscle and brain tissue metabolize the remaining 15%. The process of transamination and the urea cycle prevents this metabolic product from accumulating in the body. The elevated levels of nitrogenous toxins, including ammonia, in the systemic circulation of patients with CLD occur due to hepatocellular dysfunction and/or portosystemic shunting. This hyperammonemia is compounded by reduced peripheral metabolism of ammonia by muscle as a consequence of cachexia and muscle atrophy. Astrocytes synthesize glutamine excessively in the setting of hyperammonemia, resulting in astrocyte swelling and the generation of reactive oxygen species. Astrocyte swelling, free radical generation, and increased inhibitory function of gamma-Aminobutyric Acid result in cerebral dysfunction.^1,2 HE manifests as a broad spectrum of neurological or psychiatric abnormalities ranging from subclinical alterations to coma and was commonly graded on the West Haven Criteria (WHC) of 0 to 4 (Table).³ The Grade 0 from the previous WHC, referenced in many trials included in this article, has been replaced with minimal HE in the newly updated WHC by the American Association for the Study of Liver Diseases and the European Association for the Study of the Liver.^4,5

The ammonia hypothesis posits that ammonia is key in the pathogenesis of HE.^6-10 Some of the common precipitants of HE—gastrointestinal bleeding, infection, and renal failure—promote hyperammonemia.¹¹ HE is treated with nonabsorbable disaccharides (lactulose and lactitol) and rifaximin, which reduce the serum concentration of ammonia. Given these associations between HE and ammonia, physicians have for decades tested serum ammonia levels to diagnose HE and chart its resolution. In a study conducted by the Bavarian Society of Gastroenterology,¹² 60% of the respondents to an anonymous questionnaire regularly performed ammonia analysis in all their patients with liver cirrhosis, believing that it efficiently diagnosed HE.

Accuracy of Serum Ammonia

Multiple factors affect the accuracy of ammonia levels. First, fist clenching or the use of a tourniquet during the process of phlebotomy can falsely increase ammonia levels.¹³ Second, some authors have argued that the source of the ammonia sample matters. Kramer et al.¹⁴ reported that partial pressure of ammonia correlated closely with the degree of clinical and electrophysiological abnormalities of HE. However, Nicolao et al.¹⁵ and Ong et al.¹⁶ showed that the blood ammonia levels, whether measured by total venous, total arterial, or partial pressure methods, were equivalent. Third, ammonia levels are dependent on the time to processing of the specimen. Inaccurate results may occur if the blood sample is not immediately placed on ice after collection or if it is not centrifuged within 15 minutes of collection.^17,18