Prevalence of multimorbidity and its associations with hospitalisation or death in Japan 2014–2019: a retrospective cohort study using nationwide medical claims data in the middle-aged generation

OBJECTIVE: To describe the prevalence of multimorbidity and its associations with clinical outcomes across age groups. DESIGN: Retrospective cohort study using nationwide medical claims data. SETTING: Carried out in Japan between April 2014 and March 2019. PARTICIPANTS: N=246 671 Japanese individuals aged 20–74 enrolled in the health insurance were included into the baseline data set for fiscal year (FY) 2014. Of those, N=181 959 individuals were included into the cohort data set spanning FY2014–FY2018. EXPOSURES: Multimorbidity was defined as having ≥2 of 15 chronic conditions according to the International Classification of Diseases 10th Revision codes of the Charlson Comorbidity Index. PRIMARY AND SECONDARY OUTCOMES: Primary outcome: the standardised prevalence of multimorbidity across age groups was evaluated using data from FY2014 and extrapolated to the Japanese total population. Secondary outcome: hospitalisation or death events were traced by month using medical claims data and insurer enrolment data. Associations between multimorbidity and 5-year hospitalisation and/or death events across age groups were analysed using a Cox regression model. RESULTS: The standardised prevalence rate of multimorbidity in the nationwide Japanese total population was estimated to 26.1%. The prevalence rate with age was increased, approximately 5% (ages 20–29), 10% (30–39), 20% (40–49), 30% (50–59), 50% (60–69) and 60% (70–74). Compared with individuals aged 20–39 without multimorbidity, those with multimorbidity had a higher incidence of clinical events in any age group (HR=2.43 (95% CI 2.30 to 2.56) in ages 20–39, HR=2.55 (95% CI 2.47 to 2.63) in ages 40–59 and HR=3.41 (95% CI 3.23 to 3.53) in ages ≥60). The difference in the incidence of clinical events between multimorbidity and no multimorbidity was larger than that between age groups. CONCLUSIONS: Multimorbidity is already prevalent in the middle-aged generation and is associated with poor clinical outcomes. These findings underscore the significance of multimorbidity and highlight the urgent need for preventive intervention at the public healthcare level.