title: Validation of low-dose aspirin prescription data in The Health Improvement Network: how much misclassification due to over-the-counter use?
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: Spanish Centre for Pharmacoepidemiologic Research (CEIFE), Madrid, Spain.
authors.forename: Lucía
authors.initials: L
authors.lastname: Cea Soriano
authors.affiliation: Global Epidemiology, Bayer Pharma AG, Berlin, Germany.
authors.forename: Montse
authors.initials: M
authors.lastname: Soriano-Gabarró
authors.affiliation: Global Epidemiology, Bayer Pharma AG, Berlin, Germany.
authors.forename: Luis A
authors.initials: LA
authors.lastname: García Rodríguez
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D001241:Aspirin / Q000008:administration & dosage*; D002318:Cardiovascular Diseases / Q000517:prevention & control; D016208:Databases, Factual / Q000706:statistics & numerical data; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D004366:Nonprescription Drugs / Q000008:administration & dosage*; D010975:Platelet Aggregation Inhibitors / Q000008:administration & dosage*; D055553:Prescription Drugs / Q000008:administration & dosage*; D011320:Primary Health Care; D011322:Primary Prevention / Q000706:statistics & numerical data; D055502:Secondary Prevention / Q000706:statistics & numerical data; D011795:Surveys and Questionnaires; D006113:United Kingdom
chemical_list: D004366:Nonprescription Drugs; D010975:Platelet Aggregation Inhibitors; D055553:Prescription Drugs; D001241:Aspirin
keywords: low-dose aspirin; nonprescription drugs; over-the-counter drugs; pharmacoepidemiology