title: Development and verification of a "virtual" cohort using the National VA Health Information System.
abstract: BACKGROUND
  The VA's integrated electronic medical record makes it possible to create a "virtual" cohort of veterans with and without HIV infection to monitor trends in utilization, toxicity, and outcomes.
  
  
  OBJECTIVE
  We sought to develop a virtual cohort of HIV-infected veterans by adapting an existing algorithm, verifying this algorithm against independent clinical data, and finally identifying demographically-similar HIV-uninfected comparators.
  
  
  METHODS
  Subjects were identified from VA administrative data in fiscal years 1998-2003 using a modified existing algorithm, then linked with Immunology Case Registry (ICR, the VA's HIV registry) and Pharmacy Benefits Management (centralized database of outpatient prescriptions) to verify accuracy of identification. The algorithm was modified to maximize positive predictive value (PPV) against ICR. Finally, 2 HIV-uninfected comparators were matched to each HIV-infected subject.
  
  
  RESULTS
  Using a single HIV code, 30,564 subjects were identified (positive predictive value 69%). Modification to require >1 outpatient or 1 inpatient code improved the positive predictive value to 88%. The lack of confirmatory laboratory and pharmacy data for the majority of subjects with a single outpatient code also supported this change. Of subjects identified with the modified algorithm, 89% had confirmatory evidence. When the modified algorithm was applied to fiscal years 1997-2004, 33,420 HIV-infected subjects were identified. Two HIV-uninfected comparators were matched to each subject for an overall cohort sample of 100,260.
  
  
  CONCLUSIONS
  In the HAART era, HIV-related codes are sufficient for identifying HIV-infected subjects from administrative data when patients with a single outpatient code are excluded. A large cohort of HIV-infected subjects and matched comparators can be identified from existing VA administrative datasets.
authors.affiliation: VA Connecticut Health Care System, West Haven, CT 06516, USA. amy.justice2@va.gov
authors.forename: Shawn L
authors.initials: SL
authors.lastname: Fultz
authors.forename: Melissa
authors.initials: M
authors.lastname: Skanderson
authors.forename: Larry A
authors.initials: LA
authors.lastname: Mole
authors.forename: Neel
authors.initials: N
authors.lastname: Gandhi
authors.forename: Kendall
authors.initials: K
authors.lastname: Bryant
authors.forename: Stephen
authors.initials: S
authors.lastname: Crystal
authors.forename: Amy C
authors.initials: AC
authors.lastname: Justice
mesh_terms: D000328:Adult; D000368:Aged; D000465:Algorithms; D015331:Cohort Studies; D005260:Female; D006801:Humans; D007256:Information Systems; D008297:Male; D016347:Medical Records Systems, Computerized; D008875:Middle Aged; D014481:United States; D014493:United States Department of Veterans Affairs