Investigations
1st investigations to order
serum electrolytes
Test
Repeat as clinically indicated to monitor electrolyte abnormalities.
Inborn errors of metabolism may also show metabolic acidosis.
Result
variable; metabolic acidosis
serum glucose
Test
Repeat as clinically indicated to monitor hypoglycaemia.
Hypoglycaemia may be a feature at presentation, particularly in younger children (<3 years).
Inborn errors of metabolism may also show a low blood sugar.
Result
variable; may be <2.8 mmol/L (<50 mg/dL)
LFTs
Test
If hyperbilirubinaemia (bilirubin levels >51.3 micromol/L [>3 mg/dL]) is present, consider other diagnoses
Result
elevated transaminase and aminotransferase; bilirubin mildly elevated, but usually normal
serum ammonia
Test
Ammonia levels are usually at least 1.5 to 20 times elevated at the time of presentation.[18]
Only 3% of patients with ammonia levels <32 micromol/L (<45 micrograms/dL) exhibit permanent neurological sequelae, whereas 11% have neurological deficits if levels are >32 micromol/L (>45 micrograms/dL).[21]
Levels >250 micromol/L (>350 micrograms/dL) are associated with increased mortality.[19]
Levels may also be elevated in inborn errors of metabolism.
Result
elevated
PT/PTT
Test
At presentation, 50% of patients show prolongation of prothrombin time.
Result
usually prolonged
urine/serum toxicology
Test
Order as part of routine work-up in all patients.
Result
negative
urinalysis
Test
Order as part of routine work-up in all patients.
Result
variable; may show ketones and elevated specific gravity
blood gas
Test
Consider if significant altered mental status or if patient is electively intubated.
Result
mixed metabolic acidosis and respiratory alkalosis
Investigations to consider
cranial CT
Test
Consider if altered mental status is present.
Result
variable; normal or may show cerebral oedema without bleed or mass
lumbar puncture with cerebrospinal fluid analysis
Test
Consider if altered mental status is present.
Patient may have increased intracranial pressure and the risk of performing a lumbar puncture in these patients should be evaluated.
Result
normal cells, protein, and glucose (if serum glucose is normal)
EEG
Test
Consider if patient has significant altered mental status.
Result
non-specific, generalised slowing and flattening of waves
liver biopsy
Test
Consider in infants <1year old, children with recurrent episodes, familial cases, or atypical cases (lack of prodromal illness).[20]
Result
no significant inflammation; histochemical studies show abundant small fat droplets within hepatocytes; ultrastructural studies show hepatic mitochondrial changes
metabolic testing
Test
It is important to rule out inborn errors of metabolism that cause similar symptoms, particularly in younger children, children who have had similar episodes in the past, and children with a history or family history of hypoglycaemic episodes, particularly after stress.
Consultation with a metabolic specialist is recommended to direct further laboratory testing.
Result
negative for identifiable inborn error of metabolism
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