Ammonia

Ammonia is produced from amino acid catabolism, intestinal bacterial urease and renal glutaminase, and is cleared mainly by the liver through the urea cycle and by glutamine synthesis in muscle and brain. When these routes fail or are bypassed, ammonia accumulates and enters the brain, where astrocytes convert it to glutamine, causing osmotic swelling, cerebral oedema and encephalopathy. It is measured on plasma, most often by an enzymatic glutamate dehydrogenase method that follows NADPH consumption.

It is a time critical emergency test for unexplained encephalopathy, particularly in neonates and children, where a raised level indicates an inborn error of metabolism. In adults with chronic liver disease, it is not needed to diagnose hepatic encephalopathy, which is a clinical diagnosis, and a normal level in a suspected case should prompt a search for another cause.

Indications. Unexplained lethargy, vomiting, seizures, coma or encephalopathy in a neonate, infant or child, suspected urea cycle disorder, organic acidaemia or fatty acid oxidation defect, acute liver failure, valproate or asparaginase toxicity, Reye like illness, unexplained encephalopathy in an adult, hyperammonaemia after liver, lung or bone marrow transplant, and portosystemic shunting.

Specimen. The pre-analytical phase determines accuracy.

• Free flowing venous or arterial blood into lithium heparin or EDTA, with the tube filled completely and capped at once.
• No prolonged tourniquet or fist clenching, and no smoking by the patient beforehand.
• Placed on ice and delivered to the laboratory at once. The plasma is separated within 15 to 20 minutes and measured within about 30 to 60 minutes, or the plasma is frozen.
• Haemolysed samples are rejected.

Reference ranges (venous plasma, assay specific).

• Adults: about 11 to 35 micromol/L (about 15 to 50 microg/dL).
• Older children: under about 40 to 50 micromol/L.
• Neonates: under about 100 to 110 micromol/L in the first days of life. Transient hyperammonaemia in preterm infants can give much higher values.

Magnitude of elevation.

• Neonate: over 150 micromol/L suggests a urea cycle disorder or organic acidaemia, and over 300 micromol/L is a metabolic emergency.
• Child or adult with altered consciousness: over 100 micromol/L needs urgent action, and over 200 micromol/L carries a high risk of encephalopathy and brain injury.
• Acute liver failure: an arterial ammonia over about 150 micromol/L predicts cerebral oedema, and over about 200 micromol/L carries a high risk of intracranial hypertension and herniation.

Causes of raised ammonia.

• Liver: acute liver failure, cirrhosis with portosystemic shunting, congenital or surgical shunts, portal vein thrombosis, Budd-Chiari syndrome.
• Inherited: urea cycle disorders (ornithine transcarbamylase, carbamoyl phosphate synthetase 1, argininosuccinate synthetase and lyase, arginase and N-acetylglutamate synthase deficiency), organic acidaemias (propionic, methylmalonic, isovaleric), fatty acid oxidation disorders, hyperinsulinism hyperammonaemia (GLUD1), lysinuric protein intolerance, HHH syndrome.
• Drugs and toxins: valproate, asparaginase, 5-fluorouracil, topiramate, carbamazepine, salicylate toxicity, glycine from irrigation fluids.
• Infection: urease producing organisms (Proteus, Klebsiella, Ureaplasma) in an obstructed urinary tract, and hyperammonaemia syndrome after lung or bone marrow transplant.
• Other: gastrointestinal bleeding, high protein load, parenteral nutrition, ureterosigmoidostomy, bariatric surgery, severe sepsis, strenuous exercise, transient hyperammonaemia of the newborn, and Reye syndrome.

Lowered. Not clinically significant.

Interpretation with other tests.

• Respiratory alkalosis early in urea cycle disorders, and metabolic acidosis with a high anion gap and ketones in organic acidaemias.
• Normal liver enzymes with high ammonia and low or normal glucose point to a urea cycle disorder, and hypoglycaemia with high ammonia suggests fatty acid oxidation or hyperinsulinism.
• A high orotic acid suggests ornithine transcarbamylase deficiency.
• Raised ammonia with abnormal liver tests and a prolonged INR indicates liver failure.

Critical findings needing immediate action.

• Ammonia over 100 micromol/L in a neonate or child with vomiting, lethargy or seizures: urgent contact with a metabolic specialist, protein intake stopped and treatment started without waiting for a final diagnosis, since delay causes permanent brain injury.
• Ammonia rising in acute liver failure with agitation or a falling conscious level.
• Hyperammonaemia after transplantation.

Interferences and pitfalls.

• Delay, warm samples, haemolysis, prolonged tourniquet, fist clenching, smoking, exercise and contamination from sweat or cleaning agents raise the result in vitro.
• The result does not correlate well with the grade of hepatic encephalopathy, and a single level does not replace clinical assessment.
• Arterial ammonia is preferred for assessing risk of cerebral oedema, but venous samples are usually adequate.
• Hyperbilirubinaemia and lipaemia interfere with some assays, and reference ranges differ by method.
• A normal level does not exclude a metabolic disorder between crises.
• Repeat testing is needed when a result does not fit the clinical picture, and the sample handling is checked first.

Special groups.

• Neonates: ammonia is the first line test in any unexplained deterioration. Transient hyperammonaemia of the newborn occurs in preterm infants with respiratory distress, and levels can exceed 400 micromol/L.
• Children: a raised level in a child with a viral illness, vomiting and encephalopathy requires exclusion of a urea cycle disorder before the illness is attributed to infection.
• Pregnancy: acute fatty liver of pregnancy and valproate use are the main causes.
• Older adults: urinary obstruction with urease producing infection and gastrointestinal bleeding are common precipitants.

Follow up tests. Blood glucose, venous or arterial blood gas, lactate, urea and electrolytes, liver function tests, INR, plasma amino acids, urinary orotic acid, urinary organic acids, acylcarnitine profile, urine ketones, valproate and salicylate levels, urine culture, imaging for portosystemic shunt, cranial CT or MRI when cerebral oedema is suspected.