7 The diagnosis of aspirin treatment failure is simpler to diagnose on a consistent basis in everyday routine clinical practice. However, the term ‘aspirin failure’ can be conceptually misleading when recurrent events occur through except mechanisms that aspirin is not expected to influence, such as collateral failure, and when the failure is actually due to non-adherence to prescribed aspirin rather than pharmacological ineffectiveness. Although alternative antiplatelet agents are often considered,

as mentioned in prevailing expert consensus clinical practice guidelines, there is insufficient evidence on which to base a recommendation for optimal antiplatelet therapy following a stroke while on aspirin.8 The objective of this

study was to compare the effectiveness of clopidogrel vs aspirin for vascular risk reduction among patients with ischaemic stroke who were on aspirin treatment at the time of their index stroke. Methods Study design and dataset We conducted a nationwide cohort study by retrieving all hospitalised patients (≥18 years) with a primary diagnosis of ischaemic stroke between 2003 and 2009 from Taiwan National Health Insurance Research Database (NHIRD). Taiwan has launched a compulsory National Health Insurance programme since 1995, which covers 99% of the population and reimburses for outpatients, inpatient services as well as prescription drugs. All contracted institutions must file claims according to standard formats, which later transform into the NHIRD. The accuracy of diagnosis of major diseases in the NHIRD, such as stroke, has been validated.9 Study population We identified all hospitalised patients who were admitted with a primary diagnosis of ischaemic stroke (International Classification

of Diseases, Ninth Revision (ICD-9) codes 433, 434, 436) among subjects (≥18 years) encountered between 2003 and 2009. This is a nationwide study that included all available and eligible patients. We defined the first ischaemic stroke during study period as the index stroke. We retrieved the information of medications prescribed by physicians prior to index stroke among these patients from the pharmacy prescription database. Only patients with ischaemic stroke who received continuous aspirin treatment ≥30 days before the index stroke were included in our study cohort. The Charlson index was used as AV-951 a measure for overall severity of comorbidities for index stroke.10 Comorbidities were confirmed by ICD-9 codes based on the diagnoses of hospitalisation for index stroke. We excluded patients with atrial fibrillation, valvular heart disease or coagulopathy, since anticoagulants, rather than antiplatelet agents, are generally more suitable for secondary stroke prevention among these patients. Information regarding patients’ medications during the follow-up period was retrieved from the pharmacy prescription database.