ABMGG Medical Biochemical Genetics common mistakes
ABMGG Medical Biochemical Genetics · Understand the traps and how to avoid them.
When a newborn-screen acylcarnitine profile shows elevated C8 and C8/C10 ratio, the candidate reflexively reports MCAD deficiency without checking whether the sample was collected after a transfusion, TPN, or in a premature infant, and without confirming with a follow-up specimen and urine organic acids.
Candidates memorize the 'textbook' pattern-to-disease mapping and treat screening results as diagnostic. They forget that newborn screening is a risk-detection tool with false positives from prematurity, parenteral nutrition, maternal carnitine status, and sample timing.
What to do instead
For every abnormal screen, explicitly list pre-analytic confounders (feeding/TPN status, transfusion, gestational age, sample age) and state the confirmatory tier (repeat dried blood spot, plasma acylcarnitines, urine organic acids, enzyme assay, molecular testing) before naming a diagnosis.
On a case of hyperammonemia, the candidate orders or interprets plasma amino acids and urine organic acids but fails to draw a simultaneous plasma ammonia, blood gas, glucose, and lactate, so they cannot distinguish a urea cycle defect from organic acidemia and cannot calculate the anion gap or assess respiratory alkalosis.
Candidates compartmentalize tests by 'which disease' rather than by the acute metabolic emergency algorithm, and they under-appreciate that ammonia is exquisitely pre-analytic (delayed processing falsely elevates it).
What to do instead
Use a fixed hyperammonemia panel drawn together (ammonia, gas, glucose, lactate, plasma amino acids, urine organic acids, acylcarnitines) and always interpret ammonia alongside the acid-base and anion-gap picture; note processing time requirements.
The candidate attributes an elevated phenylalanine with a normal or only mildly elevated tyrosine to classic PKU without checking the phenylalanine/tyrosine ratio, BH4 status, or pterin profile, and without considering dietary protein load or liver disease.
Single-analyte pattern recognition. Candidates do not systematically apply the ratio-based and cofactor-based reasoning that separates PKU from BH4 defects and from secondary hyperphenylalaninemia.
What to do instead
For hyperphenylalaninemia, always compute the Phe/Tyr ratio, review whether tyrosine is low (suggesting a block) versus normal/high, and specify when pterin analysis/DHPR testing or a BH4 loading test is indicated.
When asked to manage an acutely decompensated patient with a suspected fatty acid oxidation defect, the candidate recommends a normal feeding schedule or a high-carbohydrate/low-fat diet without addressing the need to avoid fasting, provide continuous glucose, and withhold carnitine until the diagnosis is clarified.
Candidates learn diets as static 'treatment for disease X' and do not reason about the physiologic defect (impaired fasting tolerance, accumulation of toxic acylcarnitines) that dictates the acute management.
What to do instead
For each disorder class, derive management from pathophysiology: for FAO defects, avoid fasting, give IV glucose to suppress lipolysis, and be cautious with carnitine (use only for specific indications such as carnitine depletion or certain intoxications). State the rationale.
The candidate treats an elevated blood spot C3 or C5-OH as diagnostic of propionic or methylmalonic acidemia and does not request urine organic acids or plasma amino acids to distinguish the two, nor does the candidate check for secondary elevations from maternal vitamin B12 deficiency or valproate.
Candidates rely on acylcarnitine shorthand and skip the confirmatory metabolite pattern that localizes the block (methylmalonic acid vs. methylcitrate vs. propionylglycine) and skip medication/maternal confounders.
What to do instead
Always pair acylcarnitines with urine organic acids and plasma amino acids, name the discriminating metabolites, and screen for drug and nutritional confounders before assigning a diagnosis.
The candidate interprets a variant of uncertain significance in a metabolic gene as pathogenic and recommends a diagnosis and treatment plan, or conversely dismisses a known pathogenic variant because the biochemical phenotype is mild.
Candidates conflate analytic validity with clinical validity and do not apply ACMG/AMP variant-classification reasoning or genotype-phenotype correlation (e.g., mild versus severe alleles in MCAD, PKU, or MMA).
What to do instead
For any variant, state the classification evidence (population frequency, computational predictors, functional data, segregation) and integrate it with the biochemical phenotype; when uncertain, recommend functional testing and genetic counseling rather than committing to a diagnosis.
In a counseling scenario for a family with an autosomal recessive metabolic disorder, the candidate gives a recurrence risk without addressing the partner's carrier status, consanguinity, or the possibility of a de novo or mild allele, and does not discuss reproductive options or the child's prognosis in plain language.
Candidates memorize '25% for AR' and stop there, neglecting that the actual risk depends on the partner's carrier probability and that counseling requires bidirectional information (risk, options, psychosocial context).
What to do instead
Structure counseling answers as: establish the proband's genotype, estimate partner carrier risk from population frequency and ethnicity/consanguinity, compute the couple-specific risk, then present reproductive options and prognostic information in accessible terms, documenting understanding.
The candidate recommends population newborn screening for a condition without evaluating whether it meets Wilson-Jungner criteria (natural history, treatability, test performance, harms of false positives), or recommends adding a secondary target without discussing the ethical and resource implications.
Candidates focus on test technology and disease severity while ignoring the public-health and ethical framework that governs screening programs.
What to do instead
For any screening question, explicitly walk through the Wilson-Jungner/ACMG criteria: important condition, accepted treatment, facilities for diagnosis and treatment, recognizable latent stage, suitable test, and acceptable cost/harms; then discuss false-positive burden and consent.
The candidate orders a broad 'metabolic panel' or exome sequencing as a first step for a child with developmental delay and episodic vomiting, rather than a targeted tiered workup (ammonia, lactate, amino acids, organic acids, acylcarnitines, urine glycosaminoglycans) guided by the history and timing of symptoms.
Candidates equate thoroughness with test volume and do not reason about pre-test probability, test characteristics, or the diagnostic yield of tiered biochemical testing before genomic testing.
What to do instead
Justify each test by the clinical hypothesis and the phase of illness (acute vs. well), state the expected result pattern, and reserve broad genomic testing for cases where the biochemical workup is non-diagnostic or a specific syndrome is suspected.
The candidate manages a patient with a urea cycle defect or organic acidemia using only protein restriction and does not address nitrogen-scavenging therapy (sodium phenylbutyrate, sodium benzoate), arginine/citrulline supplementation, or the need for emergency protocols and sick-day rules.
Candidates treat 'diet' as the whole of metabolic therapy and do not integrate pharmacologic nitrogen disposal, cofactor supplementation, and family-facing emergency planning into chronic management.
What to do instead
For each disorder, construct a management plan with three columns: substrate restriction, pharmacologic/cofactor therapy with mechanism, and emergency/sick-day protocol including when to present to the ED; include monitoring parameters (ammonia, amino acids, growth, neurodevelopment).