Objectives: In Japan, an annual health check-up and health promotion guidance programme was established in 2008 in accordance with the Act on Assurance of Medical Care for the Elderly. A self-reported questionnaire on medication use is a required item in this programme and has been used widely, but its validity has not been assessed. The aim of this study was to evaluate the validity of this questionnaire by comparing self-reported usage to pharmacy insurance claims. Setting: This is a population-based validation study. Self-reported medication use for hypertension, diabetes and dyslipidaemia is the evaluated measurement. Data on pharmacy insurance claims are used as a reference standard. Participants: Participants were 54 712 beneficiaries of the National Health Insurance of Chiba City. Primary and secondary outcome measures: Sensitivity, specificity and κ statistics of the self-reported medication-use questionnaire for predicting actual prescriptions during 1 month (that of the checkup) and 3 months (that of the check-up and the previous 2 months) were calculated. Results: Sensitivity and specificity scores of questionnaire data for predicting insurance claims covering 3 months were, respectively, 92.4% (95% CI 91.9 to 92.8) and 86.4% (95% CI 86.0 to 86.7) for hypertension, 82.6% (95% CI 81.1 to 84.0) and 98.5% (95% CI 98.4 to 98.6) for diabetes, and 86.2% (95% CI 85.5 to 86.8) and 91.0% (95% CI 90.8 to 91.3) for dyslipidaemia. Corresponding κ statistics were 70.9% (95% CI 70.1 to 71.7), 77.1% (95% CI 76.2 to 77.9) and 69.8% (95% CI 68.9 to 70.6). The specificity was significantly higher for questionnaire data covering 3 months compared with data covering 1 month for all 3 conditions. Conclusions: Self-reported questionnaire data on medication use had sufficiently high validity for further analyses. Item responses showed close agreement with actual prescriptions, particularly those covering 3 months.
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