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 patients with stable liver disease, an ammonia level may be ordered, along with other liver function tests, when a patient suddenly “takes a turn for the worse” and becomes more acutely ill.

Significantly increased concentrations of ammonia in the blood indicate that the body is not effectively metabolizing and eliminating ammonia but do not indicate the cause. In infants, extremely high levels are 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 levels may rise and fall without causing detectible symptoms.

Increased ammonia levels and decreased glucose levels may indicate the presence of Reye’s syndrome in symptomatic children and adolescents. Increased concentrations may also indicate a previously undiagnosed enzymatic defect of the urea cycle. In children and adults, elevated ammonia levels 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 patient 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 patient symptoms to ammonia blood levels difficult.

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.

Drugs that can increase ammonia include: alcohol, barbiturates, diuretics, valproic acid, and narcotics.

Smoking

Decreased levels of ammonia may be seen with hypertension and 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.