Abstract
<title>Abstract</title> <p> <bold>Background</bold> Semantic interoperability is essential for enabling the secondary use of clinical data in research and healthcare. International standards such as SNOMED CT and LOINC are key enablers, yet empirical evidence on their real-world adoption in Switzerland remains limited. <bold>Objective</bold> This study presents a nationwide analysis of terminology usage based on metadata collected from hospitals contributing to the SPHN Federated Clinical Routine Dataset. The dataset includes information of approximately 800 000 patients, with a minimum data coverage spanning 2018 to 2025 across six Swiss University Hospitals. The goal is to quantify the adoption of semantic standards, identify strength and gaps in their usage, and highlight areas requiring further harmonization to support data quality and interoperability. <bold>Methods</bold> We analyze the use of SNOMED CT and LOINC international standard terminologies across participating hospitals by querying aggregated metadata from the SPHN Metadata Catalog via the SPARQL endpoint. <bold>Results</bold> A total of 6 557 SNOMED CT and 3 693 LOINC distinct codes were identified, representing approximately 717 million data instances. SNOMED CT and LOINC accounted for 60% and 32% of coded data instances, respectively. The remaining code instances originated from other coding systems (e.g., ATC, ICD-10-GM, CHOP). Despite this adoption, substantial heterogeneity was observed across hospitals, with most codes being institution-specific. A greater diversity of distinct codes was observed in domains such as “Substance” and “Organism”, whereas frequently instantiated concepts were concentrated in “Clinical Finding”, “Procedure”, and “Qualifier Value” hierarchies. LOINC code usage was dominated by routine laboratory tests across all institutions but with little overlap in the coding practice across institutions. <bold>Conclusions</bold> While SNOMED CT and LOINC are widely used within the SPHN framework, significant variability in code usage and limited cross-institutional overlap highlight ongoing challenges for semantic harmonization. This study provides the first nationwide empirical baseline of terminology adoption in SPHN and demonstrates the value of metadata-driven approaches for monitoring interoperability readiness. The findings support targeted efforts to improve standardization and enhance the reuse of clinical data for research. </p>