Clinical and Laboratory Diagnosis of Inborn Errors of Metabolism
Framing the diagnostic problem
Inborn errors of metabolism (IEMs) present as symptom clusters, not as named diseases: encephalopathy with hyperammonemia, hypoglycemia without ketosis, metabolic acidosis with an elevated anion gap, hyperammonemia with respiratory alkalosis, cardiomyopathy, hepatosplenomegaly, or a dysmorphic/neurodegenerative course. The candidate must be able to move from a clinical pattern to a first-tier biochemical panel (ammonia, blood gas, lactate, glucose, ketones, plasma amino acids, acylcarnitine profile, urine organic acids, homocysteine, and urine glycosaminoglycans/organic acids as indicated) before ordering expensive molecular testing.
Key concepts to master
- Pre-analytic handling is diagnostic. Ammonia must be drawn from a free-flowing line, transported on ice, and separated promptly; a hemolyzed or delayed sample falsely elevates ammonia. Acylcarnitines and amino acids require appropriate collection timing relative to feeds.
- Provoked vs. basal testing. A metabolic disorder may be invisible when the patient is well. Testing during an acute crisis, or with a controlled diagnostic fast/provocation under supervision, is more informative.
- Pattern recognition over single analytes. An elevated anion gap with normal lactate suggests organic acidemia; hyperammonemia with a normal anion gap and low citrulline suggests a proximal urea cycle defect; hyperammonemia with an elevated anion gap suggests an organic acidemia.
- Confirmation logic. Biochemical screening identifies a pattern; confirmatory testing (enzyme assay, molecular testing, or both) establishes the diagnosis and guides family testing.
Traps
Questions often bait with a single abnormal value in a well-appearing child, or with a normal newborn screen in a child who later decompensated. Remember that newborn screening is a screening test, not a diagnostic test, and that some disorders (e.g., certain urea cycle defects, mild organic acidemias) may be missed or present after the screening window.
Rule of thumb
When hyperammonemia is present, calculate the anion gap and review the plasma amino acids before ordering molecular testing: the gap plus the citrulline/arginine pattern usually narrows the differential to a urea cycle defect, an organic acidemia, or a fatty acid oxidation defect.