Clinical background:
Measurement of ammonia should be a first line biochemical investigationin the acutely ill neonate/infant and also in cases of unexplained altered mental state or prolonged vomiting without diarrhoea in patients of any age.Hyperammonaemia results from defective catabolism of amino acids to urea. Ammonia is neurotoxic and symptoms of hyperammonaemia are essentially neurological but with varying severity and age of onset.
For this reason, prompt requesting and sample analysis of plasma ammonia is essential in any patient with suspected hyperammonaemia.
The following websites include relevant guidelines:
Specimen container paediatric:
EDTA plasma
Specimen container adult:
EDTA plasma
Minimum volume paediatric:
0.5 mL blood
Minimum volume adult:
0.5 mL
Special requirements:
Please send the sample promptly to the laboratory, by hand if possible, to minimise delay in analysis.
Sample stability:
Falsely high results may be obtained if there is a delay in analysis over 2 hours.
Transport requirements:
Bring immediately to the laboratory, marked for urgent analysis.
Interpretation:
Ammonia > 200 µmol/L in neonates or > 150 µmol/L in older patients requires immediate attention. Consider contacting a Metabolic specialist.
Causes of hyperammonaemia can be split into:
Inherited metabolic disorders: including urea cycle defects, other amino acid disorders, organic acidurias, fatty acid oxidation defects
Acquired causes: including liver dysfunction and certain medications
Artefactual/other: including sample haemolysis, delayed analysis, transient hyperammonaemia of newborn.
First line investigations into hyperammonaemia requires:
Repeat plasma ammonia
Blood gases
U&E
LFT
Clotting screen
Plasma glucose
Plasma lactate
Serum/plasma calcium
Blood cultures
Urine ketones
If an inherited metabolic cause is suspected then the following tests should be requested, before initiation of treatment if at all possible:
Urine organic acids
Urine and plasma amino acids
Urine orotic acid
Blood spot acylcarnitines
Reference ranges:
0 – 4 week: 0 – 100 µmol/L
4 weeks and older: 0 – 50 µmol/L