Understanding end-of-life multimorbidity: An analysis of multiple causes of death in Denmark

Background: Mortality analysis traditionally focuses on the single underlying cause of death (UCD), which obscures the wider morbidity process at the end of life. Multiple causes of death (MCoD) data, recording all conditions on the death certificate, are increasingly used as a proxy for end-of-life multimorbidity, yet how accurately they represent it remains underinvestigated. We assessed whether recorded causes reflect end-of-life health conditions or rather the chain of events leading to death. Methods: Using linked Danish registers (Population, Cause of Death, Chronic Diseases, and Cancer), we studied residents aged 50+ diagnosed with COPD, dementia, diabetes, or cancer who died in 2010-2022 (ranging from 38779 to 224330 per disease cohort). We examined how often each diagnosed disease appeared on the certificate, its location and selection as the UCD, factors associated with its appearance (logistic regression), disease-specific mortality (multiple decrement life tables), and disease associations (Cause of Death Association Indicator, CDAI). Results: Cancers appeared on the death certificate far more often than chronic diseases (around 75% versus 19-58%) and were usually recorded in Part 1 and selected as the UCD, whereas chronic diseases were rarely the UCD. The odds of a disease appearing depended on factors such as age at and time since diagnosis. When a diagnosed disease was recorded, the certificate traced a coherent path to death; when it was absent, ill-defined causes became more common. The CDAI highlighted specific association pathways between diseases. Conclusions: MCoD data capture only part of the chronic disease burden present at death and should be interpreted cautiously as a proxy for end-of-life multimorbidity. They are, however, well suited to describing the pathways leading to death.