Propionyl Carnitine Metabolic Profile: Optimizing the Newborn Screening Strategy Through Customized Cut-Offs

Background: The advent of tandem mass spectrometry (MS/MS) had an essential
role in the expansion of newborn screening (NBS) for different inborn errors of metabolism
(IEMs). Nowadays, almost 50 IEMs are screened in Italy. The use of second-tier tests (2-TTs)
in NBS minimizes the false positive rate; nevertheless, the metabolic profile is influenced not
only by the genome but also by environmental factors and clinical variables. We reviewed
the MS/MS NBS data from over 37,000 newborns (of which 8% required 2-TTs) screened in
the Italian Abruzzo region to evaluate the impact of neonatal and maternal variables on
propionate-related primary biomarker levels. Methods: Expanded NBS and 2-TT analyses
were performed using MS/MS and liquid chromatography–MS/MS methods. We set up
layered cut-offs dividing all 37,000 newborns into categories. Statistical analysis was used to
create alarm thresholds for NBS-positive samples. Statistically significant differences were
found in both neonatal and maternal conditions based on the 2-TTs carried out. According
to the stratified cut-offs, only 1.47% of the newborns would have required a 2-TT while
still retaining the ability to recognize the true-positive case of methylmalonic acidemia
with homocystinuria, which has been identified by NBS. To further support the clinical
applicability, we performed an external evaluation considering nine positive cases from an
extra-regional neonatal population, confirming the potential of our model. Interestingly, the
setting of alarm thresholds and their application would allow for establishing the degree
of priority/urgency for 2-TTs. Conclusions: Tailoring NBS by customized cut-offs may
enhance the application of precision medicine, focusing on true-positive cases and also
reducing analysis costs and times.