Effect of Income Level on Stroke Incidence and Mediated Effects of Medication Adherence in Newly Diagnosed Hypertensive Patients: A Causal Mediation Analysis Using a Nationwide Cohort Study in South Korea

Objectives: This study aimed to determine the associations between income level and stroke incidence in hypertensive patients. We also tried to measure the mediating effect of poor antihypertensive medication adherence through causal mediation analysis. Methods: This study used the National Health Insurance Service National Sample Cohort Data from 2006 to 2015. The study population was newly diagnosed patients with hypertension aged 18 to 80. The associations between the independent-dependent variables, the mediator-dependent variables were confirmed with Cox proportional hazards regression models. In order to verify the mediating effect of poor hypertension medication adherence between income level and stroke occurrence, a weighting approach of causal mediation analysis method was used. Since age was the moderate variable, 18-49 years old and 50-80 years old were stratified. Results: Under 50 years of age, all the other income groups had a significantly higher risk of poor medication adherence than the highincome group. For the patients aged 50 and older, the low-income group and the Medical Aid beneficiaries had a higher risk of poor medication adherence compared to the high-income group. In the patient under 50 years of age, the risk for the stroke incidence of the low-income group and the Medical Aid beneficiaries were higher than those in the high-income group. In the patients aged 50 and older, all the other income groups had a significantly higher risk of incidence of stroke. Causal mediation analysis showed that the mediated effect of poor medication adherence in all groups was not significant. Conclusions: Low income significantly increased poor hypertension medication adherence and the incidence of stroke. The mediated effect of poor medication adherence was not significant.