Multisystem disease accumulation in routine claims reveals dynamic multimorbidity and mortality risk in older adults: a multicentre retrospective cohort study

Background Ageing research treats late-life vulnerability as accumulation of heterogeneous deficits, but multimorbidity is commonly measured as a static count. We asked whether short-term change in claims-documented disease burden captures a dynamic dimension of multimorbidity in older adults. Methods We conducted a multicentre retrospective cohort study using longitudinal claims and clinical records from 6,642 adults aged 65 years or older across five Japanese hospitals. Among 5,018 who reached a 6-month landmark, we separated Month 0 burden from six-month multisystem net accumulation and care-utilisation intensity. Associations with post-landmark mortality were assessed using Cox models and C-statistics, with facility-held-out validation of trajectory-only and utilisation-enriched models. Results The crude proportion with observed death within 12 months after the landmark ranged from 1.9% to 24.6% across low-burden/low-accumulation and high-burden/high-accumulation phenotypes in acute care, and from 7.0% to 26.4% in chronic care. Net accumulation improved discrimination beyond Charlson Comorbidity Index plus baseline burden (Delta C + 0.042 acute; +0.019 chronic). Trajectory-only models preserved risk ordering in held-out hospitals (C = 0.660 acute; 0.746 chronic). Conclusions Claims-derived multisystem accumulation captures a dynamic dimension of late-life multimorbidity that static counts miss and may provide a scalable longitudinal phenotype for ageing research when repeated frailty or functional assessments are unavailable.