ABMGG Clinical Biochemical Genetics common mistakes
ABMGG Clinical Biochemical Genetics · Understand the traps and how to avoid them.
Treating a normal newborn screening result as definitively excluding a disorder, and stopping the workup for a symptomatic infant (e.g., accepting a normal acylcarnitine profile in a decompensating neonate without repeat testing during acute illness).
Candidates over-anchor on the screening result as a gold-standard test rather than recognizing its design as a high-sensitivity, lower-specificity population filter with known false negatives in the acutely ill, in premature infants, after transfusion, and with timing issues (early discharge, parenteral nutrition).
What to do instead
State explicitly that newborn screening is a screening test, not diagnostic, and that a symptomatic patient requires diagnostic biochemical testing regardless of prior screening results. Practice framing repeat/reflex testing during the acute episode as the standard of care.
Ordering and interpreting the wrong first-tier test for a presentation (e.g., sending only plasma amino acids when urine organic acids are indicated, or plasma carnitine without acylcarnitines), and then interpreting an incomplete panel as ruling out disease.
Candidates memorize disease names without mapping each disorder to the analyte class that is actually abnormal, so the test selection is driven by the diagnosis they suspect rather than by the biochemical pathway and specimen matrix.
What to do instead
Build a pathway-to-analyte table (aminoacidopathies → plasma amino acids; organic acidemias/fatty acid oxidation → urine organic acids plus acylcarnitines; peroxisomal → very long chain fatty acids; lysosomal → enzyme activity plus substrate) and rehearse which specimen and timing is required for each.
Interpreting a single metabolite value without its context (age, fasting state, specimen timing, hemolysis, renal function), for example calling a mildly elevated phenylalanine diagnostic of PKU without confirming with a quantitative repeat and without checking for tetrahydrobiopterin deficiency or liver dysfunction.
Candidates treat reference ranges as absolute and skip the differential of an abnormal analyte, forgetting pre-analytic variables and secondary causes that mimic inborn errors.
What to do instead
For every abnormal result, verbalize the pre-analytic confounders and secondary causes before assigning a diagnosis; state the confirmatory test and the pathway-specific next step (e.g., pterin profile and DHPR testing for hyperphenylalaninemia).
Confusing the acute decompensation management with the chronic maintenance plan, or omitting the 'stop the catabolic state' principle (e.g., not providing sufficient glucose/calories during illness, or continuing a protein-restricted diet in an acutely decompensating urea cycle patient).
Candidates learn the chronic diet and the acute protocol separately and fail to recall that acute illness management is driven by reversing catabolism, which often means more calories and temporary relaxation of protein restriction, plus specific scavenger/cofactor therapy.
What to do instead
Structure answers as 'acute crisis' versus 'chronic/maintenance' and for each state name the caloric goal, protein target, and disorder-specific agents (nitrogen scavengers, carnitine, cofactors, dialysis indications).
Missing the treatable cofactor-responsive forms and the urgency of treatment before confirmation (e.g., not starting biotin, thiamine, riboflavin, pyridoxine, or tetrahydrobiopterin trials, or delaying treatment while awaiting confirmatory testing).
Candidates default to 'confirm first, treat later', which is wrong for disorders where a cofactor trial or empiric therapy prevents irreversible damage, and they under-recall which disorders have cofactor-responsive subsets.
What to do instead
Maintain a cofactor-to-disorder list and state that treatment should not be delayed for confirmation when the presentation is consistent; describe the cofactor trial design (dose, duration, biochemical and clinical endpoints).
Describing a disorder's natural history and prognosis generically without distinguishing severity, age of onset, and the effect of early treatment (e.g., treating all patients with a given enzyme deficiency as having the same outcome).
Candidates learn a single textbook phenotype and do not integrate genotype-phenotype correlation, residual enzyme activity, and the treatment-era natural history that changes prognosis.
What to do instead
For each disorder, practice stating the spectrum (severe/attenuated, infantile/juvenile/adult), the modifiers (residual activity, genotype), and how early detection and treatment shift outcomes.
Giving genetic counseling that quotes a recurrence risk without specifying the inheritance model, the assumptions behind it (carrier status of the partner, consanguinity, de novo possibility), or the residual risk after a negative test.
Candidates recite the textbook risk figure and skip the conditional logic, so the counseling is technically correct but clinically incomplete and misleading in edge cases.
What to do instead
For every counseling answer, name the inheritance pattern, state the assumptions, give the risk, and then give the modified risk when an assumption is relaxed (e.g., partner carrier probability from population frequency, or residual risk after sequencing).
Failing to specify confirmatory and orthogonal testing after a screening or enzyme result, and not recognizing when enzyme activity alone is unreliable (e.g., in the presence of transfused blood, or when a normal enzyme result in leukocytes does not exclude a disorder).
Candidates stop at the first abnormal result and do not build a confirmatory algorithm that includes molecular testing, a second specimen type, or a different methodology.
What to do instead
Practice stating, for each disorder, the screening test, the confirmatory biochemical test, and the orthogonal/molecular confirmation, with explicit notes on specimen pitfalls such as recent transfusion.
Ignoring laboratory stewardship and quality concepts: not recognizing the impact of assay interference, not knowing when to reject or repeat a specimen, and not distinguishing a false positive from a true positive in a screening program.
Candidates focus on disease knowledge and treat laboratory operations as background, so they cannot reason about turnaround, sensitivity/specificity, positive predictive value, or quality-control failures.
What to do instead
Review the definitions of sensitivity, specificity, and positive predictive value in the context of a low-prevalence screening population, and practice deciding when a result requires repeat, reflex, or rejection based on pre-analytic and analytic criteria.
Providing a management recommendation without monitoring parameters and follow-up intervals (e.g., recommending a diet but not naming the biochemical and clinical monitoring, or not specifying when to reassess).
Candidates treat treatment as a single decision rather than an ongoing loop of monitoring, so answers lack the measurable endpoints that define good metabolic management.
What to do instead
For each treatment answer, add the monitoring plan: which analytes, how often, what clinical parameters, and what triggers a change in therapy or escalation.