title: Supplementary data collection with case-cohort analysis to address potential confounding in a cohort study of thromboembolism in oral contraceptive initiators matched on claims-based propensity scores.
abstract: OBJECTIVE
  Residual confounding is a potential limitation of pharmacoepidemiologic studies, and in particular, studies based on administrative claims data that do not capture lifestyle and clinical confounders. We describe an application of the case-cohort design to assess residual confounding by thromboembolic risk factors (e.g., smoking and obesity) not captured in claims data in a claims-based cohort study of thromboembolism among matched oral contraceptive (OC) initiators.
  
  
  METHODS
  This study was conducted using the Ingenix Research Data Mart, a database containing medical claims for approximately 12 million members of a large health plan of the United States. We randomly sampled 701 OC initiators from cohorts of ethinyl estradiol/drospirenone (n = 22,429) and other OC initiators (n = 44,858) identified in the years 2001-2004 and matched by propensity score in a claims-based cohort study. Supplementary data on risk factors not measured in the cohort study were collected from medical records for the sample. We estimated the risk ratio of thromboembolism adjusted for the supplementary variables using Cox regression modified for a case-cohort design, and compared it to the rate ratio from the cohort study.
  
  
  RESULTS
  The risk ratio adjusted for the supplementary variables was 0.90 (95 per cent (%) confidence interval (CI): 0.49, 1.68) which was similar to the rate ratio (0.92; 95%CI: 0.50, 1.63), indicating negligible confounding by the supplementary variables in the cohort study.
  
  
  CONCLUSIONS
  Case-cohort methods were used to assess residual confounding in a claims-based cohort study. This approach adds to a growing number of methods to evaluate residual confounding in cohort studies.
authors.affiliation: Ingenix, i3 Drug Safety, Waltham, MA 02451, USA. patricia.eng@i3drugsafety.com
authors.forename: P Mona
authors.initials: PM
authors.lastname: Eng
authors.forename: John D
authors.initials: JD
authors.lastname: Seeger
authors.forename: Jeanne
authors.initials: J
authors.lastname: Loughlin
authors.forename: C Robin
authors.initials: CR
authors.lastname: Clifford
authors.forename: Sherry
authors.initials: S
authors.lastname: Mentor
authors.forename: Alexander M
authors.initials: AM
authors.lastname: Walker
mesh_terms: D000293:Adolescent; D000328:Adult; D016022:Case-Control Studies; D002648:Child; D015331:Cohort Studies; D015986:Confounding Factors, Epidemiologic; D003276:Contraceptives, Oral / Q000009:adverse effects*; D003627:Data Interpretation, Statistical; D016208:Databases, Factual; D004812:Epidemiologic Methods; D005260:Female; D006801:Humans; D008875:Middle Aged; D009765:Obesity / Q000150:complications; D017891:Pharmacoepidemiology / Q000379:methods*; D016016:Proportional Hazards Models; D012307:Risk Factors; D012907:Smoking / Q000009:adverse effects; D013923:Thromboembolism / Q000139:chemically induced* / Q000453:epidemiology; D014481:United States
chemical_list: D003276:Contraceptives, Oral