title: Validation of a proxy for estrogen receptor status in breast cancer patients using dispensing data.
abstract: OBJECTIVE
  To assess the performance of a proxy for estrogen receptor (ER) status in breast cancer patients using dispensing data.
  
  
  METHODS
  We derived our proxy using 167 patients. ER+ patients had evidence of at least one dispensing record for hormone therapy during the lookback period, irrespective of diagnosis date and ER- had no dispensing records for hormone therapy during the period. We validated the proxy against our gold standard, ER status from pathology reports or medical records. We assessed the proxy's performance using three lookback periods: 4.5 years, 2 years, 1 year.
  
  
  RESULTS
  More than half of our cohort (62%) were >50 years, 54% had stage III/IV breast cancer at recruitment, (46%) were diagnosed with breast cancer in 2009 and 23% were diagnosed before 2006. Sensitivity and specificity were high for the 4.5 year lookback period (93%, 95% CI: 86-96%; and 95%: 83-99%), respectively) and remained high for the 2-year lookback period (91%: 84-95%; and 95%: 83-99%). Sensitivity decreased (83%: 75.2-89%) but specificity remained high (95%: 83-99%) using the 1-year lookback period and the period is long enough to allow sufficient time for hormone therapy to be dispensed.
  
  
  CONCLUSIONS
  Our proxy accurately infers ER status in studies of breast cancer treatment based on secondary health data. The proxy is most robust with a minimum lookback period of 2 years.
authors.affiliation: Prince of Wales Clinical School and Lowy Cancer Research Centre, Faculty of Medicine, University of New South Wales; Pharmacoepidemiology and Pharmaceutical Policy Research Group, Faculty of Pharmacy, University of Sydney.
authors.forename: Preeyaporn
authors.initials: P
authors.lastname: Srasuebkul
authors.forename: Timothy A
authors.initials: TA
authors.lastname: Dobbins
authors.forename: Sallie-Anne
authors.initials: SA
authors.lastname: Pearson
mesh_terms: D001315:Australia / Q000453:epidemiology; D001943:Breast Neoplasms / Q000188:drug therapy* / Q000453:epidemiology / Q000378:metabolism* / Q000473:pathology; D015331:Cohort Studies; D005163:Factor Analysis, Statistical; D005260:Female; D006801:Humans; D008875:Middle Aged; D015233:Models, Statistical; D011960:Receptors, Estrogen / Q000378:metabolism*; D015203:Reproducibility of Results; D012189:Retrospective Studies
chemical_list: D011960:Receptors, Estrogen