Validation of low‐dose aspirin prescription data in The Health Improvement Network: how much misclassification due to over‐the‐counter use?

PURPOSE: We aimed to quantify the extent of over‐the‐counter (OTC) low‐dose aspirin use among patients in The Health Improvement Network (THIN) in the UK. METHODS: In September 2013, a random sample of low‐dose aspirin users (75 past users and 75 never users) was identified based on prescriptions recorded in THIN. Primary care practitioners (PCPs) were sent questionnaires to provide information on patients' use of OTC low‐dose aspirin. RESULTS: One hundred and forty valid questionnaires were received (93.30% [95%CI: 88.16–96.34] response rate). Current use of low‐dose aspirin was reported by PCPs in 4.23% (95%CI: 1.45–11.70) (n = 3) of past users (OTC use in one patient) and in 2.9% (95%CI: 0.78–9.70) (n = 2) of never users (OTC use in one patient). In addition, PCPs reported past use of low‐dose aspirin in 88.70% (95%CI: 79.31–94.18) (n = 63) of past users (all prescribed; none as OTC) and in 2.82% (95%CI: 0.78–9.70) (n = 2) of never users (as OTC). Among past users, PCPs reported the indication for low‐dose aspirin as primary cardiovascular disease (CVD) prevention in 63.16% (95%CI: 50.18–74.48) of patients and secondary CVD prevention in 31.58% (95%CI: 21.00–44.48) of patients. Corresponding percentages based on THIN were 78.95% (95%CI: 66.71–87.53) and 21.1% (95%CI: 12.47–33.29), respectively. CONCLUSION: Our findings show the small impact of potential misclassification of low‐dose aspirin use in THIN due to unrecorded OTC use. The small proportion of false negatives confirms the utility of THIN for utilization and outcome studies of low‐dose aspirin. © 2015 The Authors. Pharmacoepidemiology and Drug Safety published by John Wiley & Sons Ltd.