Population age structure explains the ecological association between older-adult solo household rate and infusion therapy utilization in Japan: An ecological study.

Heat-health warning systems rely on meteorological indicators, but heat outcomes may also reflect social vulnerability. We examined whether prefectural elderly solo household rate, a routinely available household-composition indicator, was associated with large-volume infusion therapy utilization, a non-specific healthcare-utilization measure, and whether this association was independent of population age structure, healthcare supply, and heat exposure. We conducted an ecological study of 47 Japanese prefectures using fiscal year 2023 data. Infusion therapy utilization (procedure code G004, ≥ 500 mL) from the National Database (NDB) Open Data was the outcome; elderly solo household rate was derived from the 2020 National Census. Associations were examined using linear regression, sensitivity analyses, and, in an additional analysis, adjustment for prefectural ageing rate, healthcare supply, and official heat-exposure measures. Elderly solo household rate was associated with infusion therapy utilization (β = 723.37; 95% CI, 376.52-1,070.21; p = 0.0001), persisting across sensitivity analyses. In an additional analysis, adjusting for prefectural ageing rate attenuated this estimate by 57.1% (β = 310.2; 95% CI, - 87.0-707.4; p = 0.126), and further adjustment for healthcare supply and heat exposure reduced it toward zero (β = -31.8; 95% CI, - 536.8-473.3; p = 0.902). Elderly solo household rate was associated with infusion therapy utilization in the unadjusted analysis, but this association was not robust to additional adjustment for population age structure, healthcare supply, and heat exposure. These findings indicate that population age structure explains most of this ecological association, underscoring the importance of accounting for demographic composition in heat-health surveillance.