Synthesizing expert consensus to define a core condition set for multimorbidity measurement: A narrative review

Inconsistent measurement of multimorbidity hinders comparative research and evidence synthesis. To address this, this study is a narrative review, informed by a structured scoping search of three major databases. We synthesized formal expert-consensus studies to identify a foundational core set of conditions that can serve as a pragmatic starting point for improving research comparability. Our synthesis of three expert consensus studies identified a comprehensive core set of 105 consistently recommended condition categories at the International Classification of Diseases, 10th revision (ICD-10) three-digit level. These conditions span 10 major clinical domains, covering key conditions such as ischemic heart disease, diabetes, dementia, and depression. To facilitate application, we mapped this core set to both ICD-10 and preliminary ICD-11 codes. A foundational consensus for measuring multimorbidity is emerging. We propose a "Core-plus-Customization" framework, where the identified core set serves as an evidence-based starting point for research, requiring local adaptation rather than being a prescriptive "minimum dataset." This operationalized core set provides a practical tool to begin standardizing measurement in the field.