A neonate with an elevated plasma ammonia concentration may have an inherited metabolic disorder. A testing protocol can help identify the underlying metabolic dysfunction.
Patient selection:
(1) neonate or young infant with clinical symptoms (failure to feed, hypothermia, somnolence, lethargy, coma, seizures or others)
(2) elevated plasma ammonia concentration
where:
• The reference defines an elevated plasma ammonia concentration as > 150 µmol/L.
• Ammonia is a difficult assay. There tends to be a wide inter-laboratory variability in results based on different methods, kit lots and other factors.
• The reference range in Tietz's Clinical Guide to Laboratory Tests gives references ranges for 0-10 days of age and 10 days to 2 years.
• Because of these factors, I will not use a specific value in the implementation.
Screening tests:
(1) anion gap (normal if <= 20 mmol/L)
(2) plasma glucose
Test Findings
Tentative Diagnosis
Further Workup
normal anion gap AND normal plasma glucose
urea cycle disorder
plasma amino acids
increased anion gap OR elevated plasma glucose
other metabolic disorder
urine organic acids
plasma amino acids
acylcarnitine profile
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