Enzymatic Evolution and Longitudinal Recovery in Biotinidase Deficiency: Genotypic and Clinical Insights from the Follow-Up of a Newborn-Screened Cohort in Emilia-Romagna, Italy

<b>Background/Objectives:</b> Biotinidase deficiency (BD) is a treatable autosomal recessive disorder included in many newborn screening (NBS) programs. The importance of early diagnosis and treatment is now well established. However, recent studies are emerging on the possibility of increased enzyme activity with age, an observation that raises questions about the long-term validity of the initial classification of these patients. This study aimed to assess the incidence, genetic and clinical features, and, notably, the longitudinal enzymatic trajectory of BD in a cohort identified by NBS in Emilia-Romagna, Italy, with implications for diagnostic re-evaluation and therapeutic decisions. <b>Methods:</b> A retrospective and prospective analysis was conducted on 64 infants recalled after NBS for suspected BD between 2016 and 2020. Biochemical, molecular, and clinical data were collected, and biotinidase (BTD) activity was monitored longitudinally. Affected individuals were supplemented with biotin and followed clinically for at least 5 years. <b>Results:</b> Thirty-one patients were diagnosed with BD (30 partial, 1 profound; incidence 1:5448). A significant and sustained increase in BTD activity was observed from diagnosis through early childhood (<i>p</i> < 0.001 up to 60 months), particularly among patients carrying the p.Asp444His variant. This enzymatic trend suggests a potential remodulation of biochemical classification over time. Genotype-phenotype concordance was high (92%), and clinical outcomes were favorable across the cohort. <b>Conclusions:</b> This study provides new evidence that BTD activity in patients with BD increases progressively, supporting the concept of age-dependent enzyme recovery. Our results support the need for systematic re-evaluation of diagnosis and treatment, especially at 12 months of age, and particularly in patients with evidence of partial activity deficiency and the p.Asp444His mutation.