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

<bold>Background: </bold>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.<bold>Objective: </bold>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.<bold>Methods: </bold>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.<bold>Results: </bold>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).<bold>Conclusions: </bold>Multimorbidity is associated with high medical expenditure in Japan. Mental diseases may contribute to increased medical costs. [ABSTRACT FROM AUTHOR]