Harmonising Definitions of Multiple Long Terms Conditions for Inflammation Research: A Co-Production Approach

Abstract Background Inflammation is implicated in many chronic diseases, but its role in driving multiple long-term conditions (MLTCs) remains unclear. The InflAIM programme aims to explore this potential link. Existing MLTC codelists are valuable but have not been designed for inflammation-focused research and often lack harmonisation across coding systems. Objective To develop a transparent, co-produced MLTC framework with harmonised codelists for epidemiological research, focusing on inflammatory conditions within the InflAIM programme. Methods Using a systematic, co-produced approach, we combined literature review, clinical consensus, and patient input. From eight sources, 363 candidate conditions were identified, refined to 60 validated MLTCs. Patients informed terminology and diagnostic experiences. Codelists were drawn from CALIBER (43), HDR UK Phenotype Library (4), MULTIPLY (8), with 7 developed de novo. Harmonisation covered Read v2, SNOMED CT, and Medcodes. Quality assurance excluded 16.5% of codes and resolved 92 overlaps. Results We produced harmonised codelists for 60 MLTCs, comprising 7,426 validated MedCodes. Clinical consensus identified 17 conditions with potential inflammatory components. Additional lists were developed for autoimmune and infectious diseases to support inflammation-focused analyses. Conclusion This framework extends existing resources to meet the needs of inflammation-focused research, offering a transparent, harmonised, and patient-informed foundation for the InflAIM programme.