The role of mental disease on the association between multimorbidity and medical expenditure

Background. Multimorbidity is the presence of two or more chronic diseases and is associated with increased adverse outcomes, including hospitalization, mortality and frequency of use of medical institutions. Objective. This study aimed to describe multimorbidity patterns, determine whether multimorbidity was associated with high medical expenditure, and determine whether mental diseases had an interaction effect on this association. Methods. We conducted a claims data-based observational study. Data were obtained for 7526 individuals aged 0–75 years from a medical claims data set for Goto, Japan, over a 12-month period (2016–17). Annual medical expenditure was divided into quintiles; the fifth quintile represented high medical expenditure. Multimorbidity status was defined as the occurrence of two or more health conditions from 17 specified conditions. Odds ratios (OR) and 95% confidence intervals (CI) for high medical expenditure were calculated by number of comorbidities. Results. In total, 5423 (72.1%) participants had multimorbidity. Multimorbidity was significantly associated with high medical expenditure, even after adjustment for age, sex and income category (OR: 10.36, 95% CI: 7.57–14.19; P < 0.001). Mental diseases had a significant interaction effect on the association between multimorbidity and high medical expenditure (P = 0.001). Conclusions. Multimorbidity is associated with high medical expenditure in Japan. Mental diseases may contribute to increased medical costs.