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



• 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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