The ammonia test is primarily used to help investigate the cause of changes in behavior and consciousness. It 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 of unknown origin or to help support the diagnosis of Reye's syndrome or hepatic encephalopathy caused by various liver diseases. An ammonia level may also be ordered to help detect and evaluate the severity of a urea cycle defect.
Some doctors 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 degree of impairment.
An ammonia test may be ordered on a newborn when symptoms such as irritability, vomiting, lethargy, and seizures arise in the first few days after birth. It may be performed when a child develops these symptoms about a week following a viral illness, such as influenza or a cold, when the doctor suspects that the child may have Reye's syndrome.
When adults experience mental changes, disorientation, sleepiness, or lapse into a coma, 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 someone suddenly "takes a turn for the worse" and becomes more acutely ill.
A significantly increased concentration of ammonia in the blood indicates that the body is not effectively metabolizing and eliminating ammonia but do not indicate the cause.
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 detectible symptoms.
An increased ammonia level and decreased glucose level may indicate the presence of Reye's syndrome in symptomatic children and adolescents. An increased concentration may also indicate a previously undiagnosed enzymatic defect of the urea cycle.
In children and adults, an elevated ammonia level may also indicate liver or kidney damage. Frequently, an acute or chronic illness will act as a trigger, increasing ammonia levels to the point that an affected person has difficulty clearing them.
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 doctors feel that arterial ammonia measurements are more clinically useful, but there is not widespread agreement on this.
This article was last reviewed on November 16, 2011. | This article was last modified on October 11, 2012.
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