| Indications for performing this test: Blood ammonia levels are primarily run in suspected cases of hepatic encephalopathy. Because bile acids are a more sensitive indicator of hepatic function, blood ammonia levels are seldom requested anymore. |
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Blood Ammonia |
| Indications for performing this test: Blood ammonia levels are primarily run in suspected cases of hepatic encephalopathy. Because bile acids are a more sensitive indicator of hepatic function, blood ammonia levels are seldom requested anymore. |
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Pathophysiology: Ammonia is a byproduct of protein catabolism, and is highly toxic to cells, especially the cells of the CNS. Fortunately, the plasma pH keeps most of the ammonia from diffusing across the blood brain barrier by converting it to the less diffusible ammonium ion. Ammonia produced by normal flora in the gastrointestinal tract is absorbed into the circulation by the portal vein. It is carried to the liver and is converted to urea for excretion by the kidneys. In a healthy animal, this process is so efficient that virtually all of the ammonia is removed from the blood on the first pass through the liver. When a breakdown in this system occurs, hyperammonemia is the result.
| The Test: This test is performed on a fasted heparinized blood sample. If the fasting sample is normal, and hyperammonemia is still suspected, the test may be repeated after the oral administration of ammonium chloride. In normal animals this will result in less than a 2.5 fold increase in the plasma ammonia concentration. Due to the volatility of ammonia in the blood, these samples need to be collected and then put on crushed ice and processed immediately. It is important to note that blood ammonia levels do not necessarily correlate well with signs of hepatic encephalopathy in all cases. |
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Causes of Hyperammonemia include:
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