From the Guidelines
The primary purpose of an ammonia blood test is to help diagnose or rule out hepatic encephalopathy (HE) in patients with liver disease, as a normal ammonia level casts doubt on the diagnosis of HE. The test measures the level of ammonia in the blood, which is a waste product normally processed by the liver. Elevated ammonia levels can indicate liver dysfunction, certain metabolic disorders, or Reye's syndrome 1.
Key Points to Consider
- Ammonia levels can be elevated in patients without manifest hepatic encephalopathy (HE) and even without liver disease, but a normal blood ammonia level has negative predictive value 1.
- The test requires a blood sample drawn from a vein, typically in the arm, and should be handled carefully and tested promptly because ammonia levels can increase if the sample sits too long.
- Normal ammonia levels typically range from 15-45 μg/dL (micrograms per deciliter) in adults, though reference ranges may vary between laboratories.
- Ammonia measurement is useful in cases of diagnostic doubt in the presence of impaired consciousness, as it can help rule out HE if the level is normal 1.
Clinical Application
In clinical practice, ammonia blood tests are used to support the diagnosis of HE in patients with liver disease, particularly when there is diagnostic uncertainty. A normal ammonia level should prompt further diagnostic work-up for other causes of delirium or encephalopathy 1. It is essential to interpret ammonia levels in the context of clinical presentation and other diagnostic findings, as elevated levels can be seen in various conditions.
Important Considerations
- Ammonia levels may remain elevated after clinical resolution of HE, and some patients with liver disease may have elevated ammonia levels without symptoms of HE 1.
- The test should be performed when the conditions for measurement are respected, and results should be interpreted in conjunction with clinical assessment and other diagnostic tests.
From the Research
Purpose of Ammonia Blood Test
The purpose of an ammonia blood test is to assess the levels of ammonia in the blood, which can be useful in diagnosing and managing various liver diseases.
- Ammonia levels are closely linked to liver disease, and hyperammonemia is associated with widespread toxicities such as cerebral edema, hepatic encephalopathy, immune dysfunction, promoting fibrosis, and carcinogenesis 2.
- The test can be used to prognosticate cirrhosis, acute liver failure, and acute on chronic liver failure 2.
- Measuring ammonia levels is important for the diagnosis of hepatic encephalopathy, the prognosis of patients with cirrhosis and liver failure 3.
- However, it is worth noting that ammonia levels may not be helpful in diagnosing, assessing severity, or determining the resolution of hepatic encephalopathy in patients with chronic liver disease 4.
Clinical Relevance
The ammonia blood test has clinical relevance in various liver diseases, including:
- Hepatic encephalopathy, where ammonia is thought to be central in the pathophysiology 5.
- Liver failure, where reduction in ammonia can be achieved by targeting its production, absorption, or elimination 5.
- Cirrhosis, where ammonia levels are associated with liver-related outcomes 3.
- Metabolic dysfunction-associated steatotic liver disease, where elevated systemic ammonia levels are uncommon 3.
Limitations
There are limitations to the laboratory assessment of hyperammonemia, despite its value in the assessment of various forms of liver disease 2.