title: Privacy-maintaining propensity score-based pooling of multiple databases applied to a study of biologics.
abstract: BACKGROUND
  A large study on the safety of biologics required pooling of data from multiple data sources, but while extensive confounder adjustment was necessary, private, individual-level covariate information could not be shared.
  
  
  OBJECTIVE
  To describe the methods of pooling data that investigators considered, and to detail the strengths and limitations of the chosen method: a propensity score (PS)-based approach that allowed for full multivariate adjustment without compromising patient privacy.
  
  
  METHODS
  The project had a central data coordinating center responsible for collection and analysis of data. Private data could not be transmitted to the data coordinating center. Investigators assessed 4 methods for pooled analyses: full covariate sharing, cell-aggregated sharing, meta-analysis, and the PS-based method. We evaluated each method for protection of private information, analytic integrity and flexibility, and ability to meet the study's operational and statistical needs.
  
  
  RESULTS
  Analysis of 4 example datasets yielded substantially similar estimates if data were pooled with a PS versus individual covariates (0%-3% difference in point estimates). Several practical challenges arose. (1) PSs are best suited for dichotomous exposures but 6 or more exposure categories were desired; we chose a series of exposure contrasts with a common referent group. (2) Subgroup analyses had to be specified a priori. (3) Time-varying exposures and confounders required appropriate analytic handling including re-estimation of PSs. (4) Detection of heterogeneity among centers was necessary.
  
  
  CONCLUSIONS
  The PS-based pooling method offered strong protection of patient privacy and a reasonable balance between analytic integrity and flexibility of study execution. We would recommend its use in other studies that require pooling of databases, multivariate adjustment, and privacy protection.
authors.affiliation: Pharmacoepidemiology and Pharmacoeconomics, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, MA 02120, USA. jrassen@post.harvard.edu
authors.forename: Jeremy A
authors.initials: JA
authors.lastname: Rassen
authors.forename: Daniel H
authors.initials: DH
authors.lastname: Solomon
authors.forename: Jeffrey R
authors.initials: JR
authors.lastname: Curtis
authors.forename: Lisa
authors.initials: L
authors.lastname: Herrinton
authors.forename: Sebastian
authors.initials: S
authors.lastname: Schneeweiss
mesh_terms: D001688:Biological Products / Q000009:adverse effects*; D003219:Confidentiality; D003627:Data Interpretation, Statistical; D006801:Humans; D013997:Time Factors; D016896:Treatment Outcome
chemical_list: D001688:Biological Products
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