title: A comparison of the recording of comorbidity in primary and secondary care by using the Charlson Index to predict short-term and long-term survival in a routine linked data cohort.
abstract: OBJECTIVE
  Hospital admission records provide snapshots of clinical histories for a subset of the population admitted to hospital. In contrast, primary care records provide continuous clinical histories for complete populations, but might lack detail about inpatient stays. Therefore, combining primary and secondary care records should improve the ability of comorbidity scores to predict survival in population-based studies, and provide better adjustment for case-mix differences when assessing mortality outcomes.
  
  
  METHODS
  Cohort study.
  
  
  METHODS
  English primary and secondary care 1 January 2005 to 1 January 2010.
  
  
  METHODS
  All patients 20 years and older registered to a primary care practice contributing to the linked Clinical Practice Research Datalink from England.
  
  
  RESULTS
  The performance of the Charlson index with mortality was compared when derived from either primary or secondary care data or both. This was assessed in relation to short-term and long-term survival, age, consultation rate, and specific acute and chronic diseases.
  
  
  RESULTS
  657,264 people were followed up from 1 January 2005. Although primary care recorded more comorbidity than secondary care, the resulting C statistics for the Charlson index remained similar: 0.86 and 0.87, respectively. Higher consultation rates and restricted age bands reduced the performance of the Charlson index, but the index's excellent performance persisted over longer follow-up; the C statistic was 0.87 over 1 year, and 0.85 over all 5 years of follow-up. The Charlson index derived from secondary care comorbidity had a greater effect than primary care comorbidity in reducing the association of upper gastrointestinal bleeding with mortality. However, they had a similar effect in reducing the association of diabetes with mortality.
  
  
  CONCLUSIONS
  These findings support the use of the Charlson index from linked data and show that secondary care comorbidity coding performed at least as well as that derived from primary care in predicting survival.
authors.affiliation: Division of Epidemiology and Public Health, Nottingham City Hospital, University of Nottingham, Nottingham, Nottinghamshire, UK.
authors.forename: C J
authors.initials: CJ
authors.lastname: Crooks
authors.affiliation: Division of Epidemiology and Public Health, Nottingham City Hospital, University of Nottingham, Nottingham, Nottinghamshire, UK.
authors.forename: J
authors.identifier: http://orcid.org/0000-0002-1135-9356
authors.initials: J
authors.lastname: West
authors.affiliation: Division of Epidemiology and Public Health, Nottingham City Hospital, University of Nottingham, Nottingham, Nottinghamshire, UK.
authors.forename: T R
authors.initials: TR
authors.lastname: Card
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D015331:Cohort Studies; D015897:Comorbidity; D016208:Databases, Factual; D003644:Death Certificates; D004739:England / Q000453:epidemiology; D005260:Female; D017052:Hospital Mortality / Q000639:trends; D006801:Humans; D008297:Male; D008875:Middle Aged; D011237:Predictive Value of Tests
keywords: EPIDEMIOLOGY; GASTROENTEROLOGY; GENERAL MEDICINE (see Internal Medicine); PRIMARY CARE; STATISTICS & RESEARCH METHODS