Health Outcomes of Patients with Distal Urea Cycle Disorders Detected by Newborn Screening: Data from the Spanish National Registry.

Urea cycle disorders (UCDs) are rare inherited metabolic diseases associated with toxic hyperammonemia, leading to severe neurological damage and early mortality. Early diagnosis of distal UCDs through newborn screening (NBS) enables presymptomatic intervention; however, comparative real-world outcome data remain limited. We conducted a retrospective, multicenter study using data from the Spanish UCD Registry to describe the clinical characteristics and compare health outcomes between patients diagnosed through NBS (n = 40) and those diagnosed after clinical presentation (n = 53). Patients identified by NBS showed a markedly more favorable clinical prognosis, with a mortality rate of 2.5% compared with 15.1% in the unscreened cohort, as well as significantly lower rates of neurological involvement, fewer hospital admissions due to metabolic decompensation, and a reduced need for liver transplantation. Screening also identified a high prevalence of argininosuccinate synthetase deficiency (ASS1D) cases with attenuated biochemical profiles, highlighting the relevance of sensitive screening cutoffs. These findings provide real-world evidence that presymptomatic diagnosis through NBS is associated with improved survival and long-term neurological outcomes in patients with distal UCDs.