At a glance

Also known as

Plasma ammonium; NH3

Why get tested?

To detect elevated levels of ammonia in the blood, to evaluate changes in consciousness, or to help diagnose hepatic encephalopathy and inborn errors of metabolism e.g. errors in the synthesis of urea.

When to get tested?

Venous or arterial ammonia is most often measured in the intensive care unit when acute liver failure is suspected. If a patient experiences mental changes or lapses into a coma of unknown origin; if an infant or child experiences confusion, frequent vomiting and increased lethargy as a newborn or about a week after a viral illness

Sample required?

A blood sample drawn from a vein or artery in your arm

Test preparation needed?

None

What is being tested?

This test measures the amount of ammonia in the blood. Most of the ammonia is produced by intestinal bacteria and by cells in the body during the digestion of protein. Skeletal muscle produces some ammonia and this may be increased by seizures or intensive exercise. A waste product, ammonia is normally transported to the liver, where it is converted into urea. The urea is then carried by the blood to the kidneys, where it is excreted in the urine. If ammonia is not efficiently converted to urea by the liver it builds up in the blood and passes through the blood/brain barrier.

In the brain, ammonia and other compounds metabolised by the liver can cause hepatic encephalopathy – mental and neurological changes that can lead to confusion, disorientation, sleepiness, and eventually to coma and even death. Infants and children with increased ammonia levels may vomit frequently, be irritable, and be increasingly lethargic. Left untreated, they may experience seizures, respiratory difficulty, and may lapse into a coma.

Problems with ammonia processing can arise from several sources, including:

  • Rare inherited defects in the urea cycle – a deficiency or defect in one or more of the enzymes necessary to complete the conversion of ammonia to urea
  • Severe liver disease – damage limits the ability of the liver to metabolise ammonia. Acute increases in ammonia may be seen in patients with stable liver disease, especially following a triggering event such as gastrointestinal bleeding or an electrolyte imbalance
  • In liver disease blood from the intestine carrying ammonia may be shunted into the general circulation without passing through the liver and being converted to urea.
  • Drugs – some drugs e.g. the antiepileptic Valproate and even salicylate may sometimes lead to increased ammonia. Salicylate is thought to be involved in Reye syndrome, a cause of increased ammonia in children.
How is the sample collected for testing?

A blood sample is obtained by inserting a needle into a vein or artery in the arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

How is it used?

The commonest cause of raised ammonia levels in the blood is acute liver failure where the levels are predicitive of severe cerebral complications. The ammonia test is primarily used to help investigate the cause of changes in behaviour 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. 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.

When is it requested?

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 and 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 especially with pre-existing liver disease. 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.

What does the test result mean?

Looking for reference ranges?

Significantly increased concentrations of ammonia in the blood indicate that the body is not effectively metabolising 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. Moderate short-lived increases in ammonia are relatively common in newborns, where the levels may rise and fall without causing detectible symptoms. Acute illness and seizures may cause raised blood ammonia levels, especially in children.

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. Not only do other wastes contribute to the changes in mental function and consciousness, but brain levels of ammonia may be much higher than blood levels, making correlation of symptoms to blood levels of ammonia difficult.

Is there anything else I should know?

Increased levels of ammonia may also be seen with:

  • Gastrointestinal bleeding – blood cells are haemolysed (broken apart) 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: valproate and salicylate
  • Smoking

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 and guidelines suggest that they are helpful for recognising hepatic encephalopathy, but there is not widespread agreement on this.

Common Questions

Is ammonia testing used to detect or monitor ammonia poisoning?

No, it is not considered clinically useful. In most cases, ammonia acts locally, burning or irritating whatever it comes in contact with but, according to the Agency for Toxic Substances and Disease Registry (ATSDR), it does not usually act as a systemic poison. Concentrated commercial ammonia, in a liquid or vapour form, causes more severe burns than the more dilute liquid household ammonia, but both can cause damage to the eyes, skin, respiratory tract, and to the mouth, throat and stomach if swallowed.

Last Review Date: October 25, 2014