Evaluating cross-country applicability of morbidity scores: validation of the Multisource Comorbidity Score in Catalonia

Multimorbidity places increasing pressure on healthcare systems, requiring effective tools to assess clinical complexity. Existing comorbidity indices are often setting-specific and lack generalizability. The Multisource Comorbidity Score (MCS), developed in Italy, has shown strong predictive value. This study aimed to externally validate MCS and to test recalibrated and context-adapted versions to enhance its performance in a different healthcare system. A longitudinal observational study included 198 753 residents aged >= 50 in the Barcelona-Esquerra health district, followed between 2016 and 2019. The original MCS was validated, and two adapted versions were tested: a recalibrated MCS with locally derived weights and an enhanced MCS incorporating primary care data. Predictive validity for 1-year mortality (primary outcome) and secondary outcomes (4-year mortality, hospitalizations, and healthcare use) was assessed using the Area Under the Receiver Operating Characteristic (AUROC) curve, survival analysis, and net reclassification improvement (NRI). All MCS versions showed good discrimination. AUROCs for 1-year mortality were 0.742 (original), 0.756 (recalibrated), and 0.771 (enhanced). Adapted versions achieved better risk reclassification and higher discrimination for long-term mortality. Higher MCS scores were associated with progressively lower survival probabilities and increased healthcare resource utilization. The MCS demonstrated satisfactory external validity in the validation context, with adapted versions offering modest improvements.