An HL7 FHIR® IG for lifestyle medicine in learning health systems: Multi-vendor wearable interoperability with documented terminology gaps

Background: Consumer wearable devices collect clinically useful physiological data from patients, yet less than 5% of this patient-generated health data (PGHD) reaches clinical systems. The European Health Data Space (EHDS) Regulation 2025/327 mandates standardized health data exchange, creating urgent need for validated transformation frameworks. Objective: To develop and validate a FHIR® R4 Implementation Guide enabling multi-vendor wearable data transformation for lifestyle medicine, documenting terminology gaps across seven vendor ecosystems. Methods: We developed FHIR® profiles, extensions, and ConceptMaps using a multi-layer architecture covering eleven domains (six lifestyle medicine pillars and five wearable-specific). Technical validation included HL7 IG Publisher compilation, FHIR® Validator conformance testing, and systematic terminology verification. Results: The IG comprises 74 profiles, 50 extensions, 14 CodeSystems, 174 ValueSets, and 28 ConceptMaps (546 artifacts). A two-phase terminology audit refined 1173 custom codes to 718 without standard direct equivalents, including genuine gaps (RMSSD, pNN50), wearable-specific metrics, and granularity not yet addressed by LOINC or SNOMED CT. Build validation produces 23 errors (all IPS upstream) and 73 warnings (97% non-actionable). Of six core HRV metrics, only SDNN has a LOINC code (80404-7). The multi-layer architecture achieved 75% weighted reuse across seven vendor ecosystems. Conclusions: This work extends the HL7 Physical Activity IG’s “Temporary Codes” pattern into a semantic convergence buffer—a vendor-neutral intermediary CodeSystem with outbound ConceptMap coverage to LOINC, SNOMED CT, and OMOP enabling multi-vendor semantic convergence. The 75% weighted reuse and iterative terminology audits progressively reduce custom codes as standards mature. All profiles are vendor-agnostic and EHDS-compliant. • IPS-compatible, EHDS-ready FHIR® IG for patient-generated wearable data. • Multi-vendor semantic convergence buffer spanning 7 wearable vendors and 11 domains across lifestyle medicine and wearable biometrics. • Documents critical wearable terminology gaps: RMSSD and pNN50 lack LOINC codes; all SNOMED CT HRV concepts are fetal/obstetric. • 718 custom codes with 28 ConceptMaps providing declarative migration paths to LOINC, SNOMED CT, and OMOP via OCL governance. • Iterative terminology audit: 1173 codes refined to 718, with 19 LOINC substitutions across recurrent audit cycles.