Table 1.
Screening targets and associated variants
| Classic form | Description | Laboratory results | Variant form |
|---|---|---|---|
| Phenylketonuria (PKU) | Deficiency of phenylalanine hydroxylase, an enzyme that converts phenylalanine into tyrosine. Dissimilar from PAH cofactor (tetrahydrobiopterin, BH4) deficiency. Phenylalanine (Phe) accumulates and can cause permanent intellectual disability. Treated with Phe-free diet. | Phenylalanine >600 μmol/l on dried bloodspot (DBS). High phenylalanine:tyrosine ratio. BH4 deficiencies excluded on DBS. Molecular genetic testing is not essential but may help determine residual activity and severity of phenotype. 1101 known PAH variants. | Mild hyperphenylalaninemia (MHP): elevated Phe below diagnostic threshold for PKU. Diagnostic uncertainty due lack of universal threshold. Therapeutic uncertainty caused by lack of consensus and little evidence for Phe-free diet. |
| Profound biotinidase deficiency | A neurocutaneous disorder caused by insufficient endogenous biotin production. This can cause seizure, respiratory problems, developmental delay, ataxia, eczema, hearing loss, vision problems, and alopecia. Treated with biotin supplements. | Low enzyme activity measured from DBS. <10% of activity on confirmatory serum test. Molecular genetic testing for biallelic pathogenic BTD variants may resolve ambiguous enzymatic tests. | Partial biotinidase deficiency: 10–30% serum biotinidase activity. Prognostic uncertainty whether natural history is symptomatic. Lower dose biotin supplementation is generally recommended. |
| Medium- and very long–chain acyl CoA dehydrogenase deficiencies (MCADD and VLCADD) | Fatty acid oxidation deficiencies prevent energy production from lipids when ill, fasting, or during muscular exercise. MCADD is associated with hypoketotic hypoglycemia and hyperammonemia causing neurological damage and hepatomegaly. VLCADD deficiency has three phenotypes: severe cardiomyopathy, adult-onset muscular problems, or late-onset hypoketotic hypoglycemia similar to MCADD. Untreated, VLCADD and MCADD have a high risk of death. | MCADD screening on DBS shows elevated C8 acylcarnitine with lower elevations in C6 and C10. VLCADD screening on DBS shows increased C14:1 acylcarnitine. Plasma acylcarnitines and urine organic acid analysis are used for confirmatory testing. Molecular genetic testing for variants in the ACADM (MCADD) or ACADVL (VLCADD) genes is useful in confirming a diagnosis. |
Variant MCADD: Diagnostic, prognostic, and therapeutic uncertainty due to residual (>10%) MCAD enzyme activity and variants of unknown significance (VUS). Variant VLCADD: Therapeutic uncertainty due to insufficient data to achieve consensus for infants at risk, degree of long-chain triglyceride dietary restriction, and medium-chain triglyceride dietary supplementation. |
| 3-Methylcrotonyl-CoA carboxylase deficiency (3-MCCD) | An inborn error of leucine catabolism that is associated with ketotic hypoglycemia, Reye syndrome, and intellectual disability. Oral carnitine may be prescribed if C5-OH levels are low. | DBS screening shows elevated C5-OH. Confirmatory tests include urine organic acids with MCC characteristic pattern and high plasma C5-OH assay. Molecular genetic testing is optional and is not always prognostically useful. |
Asymptomatic 3-MCCD: As few as 4–5% of cases are symptomatic, causing diagnostic, prognostic, and therapeutic uncertainty. There is a lack of consensus for dietary modification. Ontario recently removed 3-MCCD from the NBS panel. |
NBS newborn screening.