Ammonia

Liver Function · also called NH3, Plasma ammonia

What it measures

Ammonia is a toxic nitrogen waste normally converted to urea by the liver. Plasma ammonia is measured when hepatic encephalopathy, Reye syndrome or an inherited urea-cycle disorder is suspected.

Specimen and preparation

Specimen: EDTA or lithium-heparin plasma, free-flowing sample
Tube: Lavender top
Preparation: No tourniquet or fist clenching if possible; fill tube completely, place on ice and separate within 30 minutes. Smoking and delay falsely raise results.

Reference range

Group	Range
Adult	15–45 (11–32) µg/dL (µmol/L)
Newborn	Up to 110 (<80) µg/dL (µmol/L)

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

Liver failure and hepatic encephalopathy
Urea-cycle enzyme defects
Reye syndrome
Valproate therapy
Gastrointestinal bleeding in cirrhosis
Delayed or haemolysed sample (artefact)

Causes of low values

Not clinically significant

Method

Enzymatic assay (glutamate dehydrogenase, NADPH)

Good to know

The µg/dL figures are ammonia-nitrogen (µmol/L × 1.4); for NH₃ itself multiply µmol/L by 1.7. Levels do not correlate closely with the severity of encephalopathy; handling errors are the commonest cause of a high result.

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