The ammonia test is used to detect an elevated level in the blood that can be caused by severe liver disease, kidney failure, Reye syndrome or a rare genetic disorder of the urea cycle, for example. The test may be used to help investigate the cause of an individual's changes in behavior and consciousness.
Ammonia is a waste product naturally produced in the body. It primarily comes from the digestion of protein by bacteria in the intestines. If not processed by the liver and cleared from the body appropriately, excess ammonia can accumulate in the blood and pass from the blood into the brain, where it is toxic.
An ammonia test may be ordered, along with other tests such as glucose, electrolytes, and kidney and liver function tests, to help diagnose the cause of a coma or to help support the diagnosis of Reye syndrome or hepatic encephalopathy caused by various liver diseases. An ammonia level may also be ordered to help diagnose a rare urea cycle defect and to evaluate the severity of the condition.
Some healthcare practitioners use the ammonia test to monitor the effectiveness of treatment of hepatic encephalopathy, but there is not widespread agreement on its clinical utility. Since hepatic encephalopathy can be caused by the build-up of a variety of toxins in the blood and brain, blood ammonia levels correlate poorly with the severity of the condition.
An ammonia test may be ordered when a newborn has symptoms that arise in the first few days after birth, such as:
An ammonia test may be performed when a child develops these same symptoms about a week following a viral illness, such as influenza or chickenpox, or when a healthcare practitioner suspects that the child may have Reye syndrome.
When adults experience mental changes, disorientation, sleepiness, or lapse into a coma and may have liver disease or kidney failure, an ammonia level may be ordered to help evaluate the cause of the change in consciousness. In people with stable liver disease, an ammonia level may be ordered, along with other liver function tests, when a person suddenly becomes more acutely ill.
A significantly increased concentration of ammonia in the blood indicates that the body is not effectively processing and eliminating ammonia and it may be the cause of the person's signs and symptoms.
In infants, an extremely high level is associated with an inherited urea cycle enzyme deficiency or defect but may also be seen with hemolytic disease of the newborn. Moderate short-lived increases in ammonia are relatively common in newborns, where the level may rise and fall without causing noticeable symptoms. (For more about urea cycle disorders, see the Related Pages tab.)
An increased ammonia level and decreased glucose level may indicate the presence of Reye syndrome in symptomatic children and teens. An increased concentration may also indicate a previously undiagnosed enzymatic defect of the urea cycle.
In children and adults, an elevated ammonia level may indicate that severe liver or kidney damage has impacted the body's ability to clear ammonia and that the brain may be affected. Frequently, an acute or chronic illness will act as a trigger, increasing ammonia levels to the point that an affected person has difficulty clearing the ammonia.
A normal blood ammonia level may mean that a person's signs and symptoms are due to a cause other than excess ammonia. However, normal concentrations of ammonia do not rule out hepatic encephalopathy. Other wastes can contribute to changes in mental function and consciousness, and brain levels of ammonia may be much higher than blood levels. This can make correlation of a person's symptoms to ammonia blood levels difficult.
A decreased level of ammonia may be seen with some types of hypertension, such as essential (high blood pressure of unknown cause) and malignant (very high blood pressure that occurs suddenly and quickly).
Increased levels of ammonia may also be seen with:
Gastrointestinal bleeding - blood cells are hemolyzed in the intestines, releasing protein.
Muscular exertion - muscles produce ammonia when active and absorb it when resting.
Tourniquet use - ammonia levels can be increased in the blood sample collected.
Use of certain drugs, including alcohol, barbiturates, diuretics, valproic acid, and narcotics
Decreased levels of ammonia may be seen with the use of some antibiotics, such as neomycin.
Ammonia tests can also be performed on arterial blood, but this method is much less frequently used. Some healthcare practitioners feel that arterial ammonia measurements are more clinically useful, but there is not widespread agreement on this.
This article was last reviewed on February 29, 2016. | This article was last modified on February 29, 2016.
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