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What is the utility of measuring the serum ammonia level in patients with altered mental status?

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If you already know that the patient with altered mental status has decompensated liver disease, measuring the arterial or venous ammonia level has little utility. In these patients, one’s clinical suspicion is the main guide to diagnosing hepatic encephalopathy, and a normal or modestly elevated blood ammonia level does not rule out the diagnosis.

On the other hand, provided that it is appropriately performed, blood ammonia testing may be helpful if there is no clear evidence of underlying chronic liver disease. In these cases, an elevated blood ammonia level may have significant prognostic value (as in acute liver failure) or may prompt you to initiate further evaluation for uncommon but significant meta bolic disorders such as urea cycle disorders.

WHEN AMMONIA LEVELS RISE

Ammonia is derived predominantly from protein degradation. Most of the ammonia in the blood comes from the intestine, where colonic bacteria use ureases to break down urea into ammonia and carbon dioxide. Fortunately, blood from the intestine is carried directly to the liver via the portal vein, where 85% of the ammonia is converted back into urea, which is less toxic and is excreted by the kidneys and colon. Table 1 summarizes ammonia metabolism and the organs involved.

Ammonia levels are elevated in several conditions in which its production is increased (eg, in convulsive seizures with increased muscle production) or its clearance is impaired (eg, in hepatocellular dysfunction, portosystemic shunting, or both, with subsequent impaired hepatic detoxification of ammonia).

Because the blood-brain barrier is highly permeable to ammonia, the brain is exposed to excessive concentrations of it in these circumstances. In the brain, ammonia is thought to cause both functional and structural abnormalities that could explain neuropsychiatric dysfunction, often manifested as an altered mental status of variable degree.1–3

DOES THE PATIENT HAVE DECOMPENSATED LIVER DISEASE?

Physicians often measure the venous (and less often, the arterial) ammonia level while evaluating patients presenting with altered mental status. However, in many cases, this test result may be of uncertain utility—it may not have a significant impact on a specific patient’s management and, worse, it can confuse the physician regarding diagnosis. Also, the test itself is a needless expense. Therefore, we need to carefully consider whether to obtain a blood ammonia test and how to interpret the results in patients with altered mental status.

The key initial question in such patients is whether the patient is known to have decompensated liver disease with a typical clinical picture of hepatic encephalopathy.

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