title: Improved incidence estimates from linked vs. stand-alone electronic health records.
abstract: Electronic health records are widely used for public health research, and linked data sources are increasingly available. The added value of using linked records over stand-alone data has not been quantified for common conditions such as community-acquired pneumonia (CAP).
  
  
  
  Our cohort comprised English patients aged ≥65 years from the Clinical Practice Research Datalink, eligible for record linkage to Hospital Episode Statistics. Stand-alone general practice (GP) records were used to calculate CAP incidence over time using population-averaged Poisson regression. Incidence was then recalculated for the same patients using their linked GP-hospital admission data. Results of the two analyses were compared.
  
  
  
  Over 900,000 patients were included in each analysis. Population-averaged CAP incidence was 39% higher using the linked data than stand-alone data. This difference grew over time from 7% in 1997 to 83% by 2010. An increasingly larger number of pneumonia events were recorded in the hospital admission data compared to the GP data over time.
  
  
  
  Use of primary or secondary care data in isolation may not give accurate incidence estimates for important infections in older populations. Further work is needed to establish the extent of this finding in other diseases, age groups, and populations.
authors.affiliation: Department of Infectious Disease Epidemiology, Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK. Electronic address: elizabeth.millett@ndm.ox.ac.uk.
authors.forename: Elizabeth R C
authors.initials: ER
authors.lastname: Millett
authors.affiliation: Department of Non-communicable Disease Epidemiology, Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
authors.forename: Jennifer K
authors.initials: JK
authors.lastname: Quint
authors.affiliation: Department of Medical Statistics, Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London, WC1E 7HT, UK.
authors.forename: Bianca L
authors.initials: BL
authors.lastname: De Stavola
authors.affiliation: Department of Non-communicable Disease Epidemiology, Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
authors.forename: Liam
authors.initials: L
authors.lastname: Smeeth
authors.affiliation: Department of Infectious Disease Epidemiology, Faculty of Epidemiology and Population Health, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK.
authors.forename: Sara L
authors.initials: SL
authors.lastname: Thomas
mesh_terms: D000368:Aged; D017714:Community-Acquired Infections / Q000453:epidemiology*; D057286:Electronic Health Records / Q000592:standards / Q000706:statistics & numerical data*; D004739:England / Q000453:epidemiology; D015340:Epidemiologic Research Design; D005260:Female; D058006:General Practice; D015577:Geriatric Assessment / Q000379:methods / Q000706:statistics & numerical data*; D006801:Humans; D015994:Incidence; D008297:Male
keywords: Aged; Cohort; Data linkage; Electronic health records; England/epidemiology; Pneumonia