Metabolic Genetics and Newborn Screening Practice

Begin a metabolic vignette by identifying the clinical setting and the biochemical pattern. Then connect those findings with a pathway. Memorizing one analyte and one diagnosis can leave you unprepared when the question adds a competing explanation or asks what the result means.

Describe the pattern before naming the disorder

Ask about age, feeding or fasting, illness, neurological change, and the laboratory findings actually supplied. Distinguish an abnormal screening marker from a confirmed diagnosis. A screening program's marker, cutoff, and follow-up pathway are not interchangeable with a diagnostic assay.

Practice case: Follow up after elevated phenylalanine

A well-appearing newborn has an elevated phenylalanine result on newborn screening. Which next step is most appropriate?

1. Repeat routine screening at the next well visit before arranging further assessment.
2. Arrange prompt metabolic follow-up and confirmatory biochemical evaluation.
3. Order PAH sequencing and defer biochemical evaluation until the report returns.
4. Confirm an elevated plasma phenylalanine level and assign PAH deficiency without evaluating other causes.

Answer: B. A screen-positive result requires follow-up even when the newborn appears well. Confirmatory evaluation includes quantitative plasma amino acids and assessment for other causes of hyperphenylalaninemia, including pterin-related disorders, with a metabolic specialist. A delays evaluation; C substitutes a molecular-first wait for the needed biochemical assessment; D establishes an etiology prematurely. The screen identifies a finding, not one definitive molecular diagnosis.

ACMG elevated phenylalanine ACT sheet

Applied example: hypoketotic hypoglycemia

A newborn develops poor feeding, lethargy, and hypoglycemia during an illness. Ketones are inappropriately low, and the acylcarnitine profile shows a prominent C8 species. The nearest biochemical alternative is another fatty acid oxidation defect, such as very-long-chain acyl-CoA dehydrogenase deficiency, but the chain-length pattern differs. GSD I can also cause fasting hypoglycemia with relatively low ketones, often accompanied by lactic acidosis. It does not explain the characteristic C8-predominant acylcarnitine pattern in this vignette. The task is to use the pattern to choose the pathway, then confirm with the appropriate diagnostic testing.

Use a pathway to explain the findings

For a fatty acid oxidation example, ask why fasting might expose a problem in energy production and how ketone production fits the case. MCAD deficiency can involve hypoketotic hypoglycemia and prominent C8 elevation. That pattern supports a pathway-based differential; it does not remove the need for confirmation.

For other cases, work through the provided glucose, ketones, acid-base findings, ammonia, amino acids, or organic acids as relevant. Avoid forcing every metabolic disorder into the same laboratory pattern.

Review with current references

Use the ACMG ACT Sheets and Algorithms for current newborn screening follow-up resources. Board preparation is not a substitute for an urgent clinical pathway when an infant is unwell.

For structured study, use EveryGene Academy and the 101 Must-Know Genetic Conditions flashcards. Pediatric learners can begin with the free pediatric workbook.

Sources: ACMG ACT sheets and algorithms; MCAD deficiency GeneReviews. ACMG GSD VI and IX practice resource (2019). These are introductory teaching examples, not a complete biochemical genetics board resource.

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