Policy‐induced selection bias in pharmacoepidemiology: The example of coverage for Alzheimer's medications in British Columbia

PURPOSES: To assess the impact of a government‐sponsored reimbursement policy for cholinesterase inhibitors (ChEIs) on trends in physician visits with a diagnosis of Alzheimer's disease (AD). METHODS: Longitudinal population‐based study using interrupted time series methods. British Columbia outpatient claims data for individuals aged 65 and older were used to compute monthly AD visit rates and examine the impact of the ChEI reimbursement policy on the coding of AD. We examined trends in the number of patients with AD visits, the number of AD visits per patient, and visits with “competing” diagnoses (mental, neurological, and cerebrovascular disorders and accidental falls). Finally, we described demographic and clinical features of diagnosed patients. RESULTS: We analyzed 1.9 million AD visits. Faster growth in recorded AD visits was observed after the policy was implemented, from monthly growth of 7.5 visits per 100 000 person‐months before the policy (95% confidence interval [CI], 6.1‐8.9) to monthly growth of 16.5 per 100 000 person‐months after the policy (95% CI, 14.8‐18.3). After the implementation of the policy, we observed increased growth in the number of patients with recorded AD visits and the number of AD visits per patient, as well as a shift in diagnoses away from mental diseases and accidental falls to AD (diagnosis substitution). CONCLUSIONS: British Columbia's reimbursement policy for ChEIs was associated with a significant acceleration in Alzheimer's visits. Evaluations of health services utilization and clinical outcomes following drug policy changes need to consider policy‐induced influences on the reliability of the data used in the analysis.