Individual income and blood pressure control after antihypertensive treatment initiation accounting for regional variation: a Japanese nationwide study.

Evidence on how individual-level socioeconomic status and regional disparities jointly influence BP control after treatment initiation remains scarce. We assessed the association between income and uncontrolled BP after antihypertensive treatment initiation and examined regional variation in Japan. We conducted a cohort study using health checkup data from the Japan Health Insurance Association (2015-2022). Participants were adults aged 40-74 years who had initiated antihypertensive treatment. Income was categorized into sex-specific quintiles based on standard monthly remuneration. We estimated odds ratios (ORs) and 95% confidence intervals (CIs) for uncontrolled BP (systolic/diastolic BP ≥ 130/ ≥ 80 mmHg). We included 381 116 females and 933 876 males (mean age, 55 years). Uncontrolled BP was observed in 68.4% of females and 75.3% of males. After adjustment for covariates, the lowest income category had higher odds of uncontrolled BP than the highest category (female, OR [95% CI], 1.48 [1.45-1.51]; male, OR [95% CI], 1.26 [1.24-1.28]). This association was attenuated after adjustment for pretreatment systolic BP. Prefecture difference contributed more to model fit than income category, as evidenced by larger Akaike information criterion (AIC) increases upon removal from the fully adjusted model (ΔAIC: 551 vs 255 in females, 1433 vs. 293 in males). The income effect on uncontrolled BP remained even within each prefecture, with low-to-moderate between-prefecture heterogeneity (I² ≤ 47.7%). Lower income was associated with uncontrolled BP. This association was attenuated after adjustment for pretreatment BP, suggesting that delayed detection or treatment initiation partly explains this disparity. Meanwhile, geographic variation across prefectures remained substantial.