Ammonia

What is being measured?
This test measures the amount of ammonia in the blood. Ammonia is a compound produced by intestinal bacteria and by the body’s cells during the digestion of proteins. As a waste product, ammonia is normally carried to the liver, where it is converted into urea and glutamine. Urea is then carried in the blood to the kidneys, where it is excreted in the urine. If this “urea cycle” is not complete, ammonia builds up in the blood and crosses the blood-brain barrier.

In the brain, ammonia and other compounds normally metabolised by the liver can cause hepatic encephalopathy – mental and neurological changes that can lead to confusion, disorientation, drowsiness and eventually coma and even death. Newborns and children with high ammonia levels may vomit frequently, be irritable and become increasingly lethargic. If untreated, they may have seizures and breathing difficulties and go into a coma.

Problems with ammonia can arise from several sources, including:

Rare inherited defects of the urea cycle – a deficiency or defect in one or more of the enzymes needed to complete the conversion of ammonia into urea.
Severe liver disease – damage limits the liver’s ability to metabolise ammonia. Acute rises in ammonia can be seen in patients with stable liver disease, especially after a triggering event such as gastrointestinal bleeding or an electrolyte imbalance.
Reduced blood flow to the liver – ammonia is less able to reach the liver.
Reye’s syndrome – an acute condition affecting the blood, brain and liver. It is characterised by a rise in ammonia levels and a fall in glucose and affects children and young adults. In many cases it follows, and appears to be triggered by, a viral infection. Children who take aspirin are at increased risk.
Kidney failure – the kidneys cannot effectively eliminate urea from the body, leading to a build-up of ammonia in the blood.

How is it used?
The ammonia test is mainly used to help find the cause of changes in behaviour and awareness. It may be requested, together 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 support a diagnosis of Reye’s syndrome or hepatic encephalopathy. Ammonia may also be requested to help detect or assess the severity of a urea cycle defect.

Some doctors use the ammonia test to monitor the effectiveness of treatment for hepatic encephalopathy, but there is no general agreement on its clinical usefulness. 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 physical deterioration.

When is it requested?
An ammonia test may be requested for a newborn when symptoms such as irritability, vomiting, lethargy and seizures occur in the first days after birth. It may be done when a child develops these symptoms about a week after a viral infection such as flu or a cold and the doctor suspects the child may have Reye’s syndrome.

When adults have mental changes, disorientation or drowsiness, or slip into a coma, ammonia may be requested to help assess the cause of the change in consciousness. In patients with chronic liver disease, ammonia may be requested, together with other liver function tests, when a patient suddenly “spirals downwards” and becomes more seriously ill.

Is there anything else I should know?
Raised ammonia levels can also be seen with gastrointestinal bleeding – blood cells are broken down (haemolysed) in the intestine, releasing protein.
Muscular exertion – muscles produce ammonia when active and absorb it at rest.
Use of a tourniquet – ammonia levels may be raised in the blood sample collected.
Medicines that can raise ammonia include alcohol, barbiturates, diuretics and narcotics
Smoking.

Decreased ammonia levels can be seen with high blood pressure and the use of some antibiotics (such as neomycin).

Ammonia tests can also be performed on arterial blood, but this method is much less commonly used. Some doctors feel that arterial ammonia measurements are more clinically useful, and guidelines suggest they are useful in recognising hepatic encephalopathy, but there is no broad agreement on this.

1. Is the ammonia test 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 into 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 liquid or vapour form, causes more serious poisoning than more dilute household ammonia, but both can damage the eyes, skin and airways, and the mouth, throat and stomach if swallowed.