Newborn Screening Follow-up: A Positive Screen in a Well-Appearing Infant Whose Confirmatory Testing Is Ambiguous

Pattern: elevated C5-OH (3-hydroxyisovalerylcarnitine) with elevated C5:1, low free carnitine, plus urine 3-hydroxyisovaleric acid, 3-methylcrotonylglycine, and 3-hydroxypropionic acid, without methylmalonic acid.

Interpretation: This is the classic profile of 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency, a defect in leucine catabolism (biotin-dependent enzyme; also part of the multiple carboxylase deficiencies). The absence of methylmalonic acid and normal biotinidase argue against combined carboxylase deficiency and biotinidase deficiency. The persistent abnormality on repeat dried blood spot and confirmation in plasma acylcarnitines and a matching urine organic acid pattern makes a false positive very unlikely - three independent matrices and analytes agree.

Caveats to mention: some C5-OH elevations are transient or maternal in origin (e.g., maternal 3-MCC deficiency or maternal biotin use), so confirming the infant's own status and checking the mother's status is prudent. But the concordant urine organic acids indicate the infant is genuinely excreting the metabolites.

Key point: 3-MCC deficiency is clinically heterogeneous; many affected individuals - especially those detected by newborn screening - remain asymptomatic, and the historical severe phenotype is uncommon. Overtreatment of well infants is a real concern, but so is missing the minority who decompensate.

Approach:

1. Do not start aggressive pharmacologic or restrictive therapy reflexively in a well, growing infant. Involve a metabolic physician and dietitian.
2. Confirm and characterize: molecular testing of MCCC1/MCCC2 to confirm; check the mother's acylcarnitine/organic acids (maternal 3-MCC can cause a positive infant screen).
3. Diet: avoid excessive protein/leucine restriction; ensure adequate protein for growth. Some clinicians use modest protein moderation and carnitine supplementation, but evidence for routine carnitine in asymptomatic patients is limited; decisions should be individualized.
4. Sick-day plan: during illness/catabolism, avoid prolonged fasting and monitor for decompensation (hypoglycemia, acidosis, lethargy).
5. Monitoring: growth, development, and interval biochemical testing; watch for carnitine depletion if supplementation is used.
6. Counsel: explain the diagnosis is real but that many children do well; the goal is to prevent crises without over-restricting a growing infant. Document a shared decision and follow-up plan.

Recurrence risk: 3-MCC deficiency is autosomal recessive (MCCC1 or MCCC2). If both parents are confirmed carriers, each pregnancy has a 25% chance of being affected, 50% carrier, 25% unaffected. Confirm parental carrier status by molecular testing once the infant's variants are known.

Newborn screening interaction:

• Future pregnancies will be flagged by NBS if affected, because C5-OH is on the panel. A positive screen is not a diagnosis; it triggers confirmatory testing.
• Because the phenotype can be mild/asymptomatic, parents should understand that a positive screen does not necessarily mean severe disease, but it does warrant evaluation.
• Prenatal diagnosis is available by CVS/amniocentesis once the familial variants are known; preimplantation genetic testing is an option.
• Cascade testing of siblings and at-risk relatives should be offered.

Counseling tone: be clear that the result is real (not a lab error) but that the prognosis for screen-detected, asymptomatic infants is generally good; the plan is close follow-up rather than alarm. Provide written information and connect the family with metabolic genetics and a dietitian.