Metabolic Laboratory Testing and Newborn Screening
Metabolic Laboratory Testing and Newborn Screening
This domain focuses on the appropriate use and interpretation of laboratory tests in metabolic genetics. You must know the principles of newborn screening (NBS), including tandem mass spectrometry (MS/MS) for amino acids and acylcarnitines, and the limitations of screening (false positives/negatives, premature infants, maternal conditions). Understand confirmatory testing: plasma amino acids, urine organic acids, acylcarnitine profile, enzyme assays, and molecular testing. Also, know the significance of sample timing, collection, and handling (e.g., fasting status, hemolysis).
Common traps include misinterpreting NBS results without considering clinical context (e.g., transient elevations in premature infants), ignoring maternal factors (e.g., maternal PKU affecting the neonate), and failing to recognize that a normal NBS does not exclude all metabolic disorders (e.g., urea cycle disorders may not be detected). Additionally, be aware of secondary elevations (e.g., elevated phenylalanine in liver disease) and the importance of follow-up testing.
A rule of thumb: always confirm an abnormal NBS with a diagnostic test, and interpret results in the context of the clinical picture. For example, an elevated C8 acylcarnitine on NBS suggests medium-chain acyl-CoA dehydrogenase (MCAD) deficiency, but confirm with plasma acylcarnitine profile and genetic testing. Also, remember that NBS for congenital hypothyroidism and cystic fibrosis are not part of metabolic screening but are often included in panels.
Key Points
- Newborn screening uses MS/MS to detect aminoacidopathies, organic acidemias, and fatty acid oxidation defects.
- False positives occur with prematurity, parenteral nutrition, and maternal conditions.
- Confirmatory tests: plasma amino acids, urine organic acids, acylcarnitine profile, enzyme assay, DNA analysis.
- Timing: NBS ideally after 24-48 hours of life; early discharge may miss cases.
- Not all metabolic disorders are detected by NBS (e.g., urea cycle disorders, some lysosomal storage diseases).