title: A systematic review of prevalence studies on multimorbidity: toward a more uniform methodology.
abstract: OBJECTIVE
  We sought to identify and compare studies reporting the prevalence of multimorbidity and to suggest methodologic aspects to be considered in the conduct of such studies.
  
  
  METHODS
  We searched the literature for English- and French-language articles published between 1980 and September 2010 that described the prevalence of multimorbidity in the general population, in primary care, or both. We assessed quality of included studies with a modified version of the Strengthening the Reporting of Observational Studies in Epidemiology checklist. Results of individual prevalence studies were adjusted so that they could be compared graphically.
  
  
  RESULTS
  The final sample included 21 articles: 8 described studies conducted in primary care, 12 in the general population, and 1 in both. All articles were of good quality. The largest differences in prevalence of multimorbidity were observed at age 75 in both primary care (with prevalence ranging from 3.5% to 98.5% across studies) and the general population (with prevalence ranging from 13.1% to 71.8% across studies). Apart from differences in geographic settings, we identified differences in recruitment method and sample size (primary care: 980-60,857 patients; general population: 1,099-316,928 individuals), data collection, and the operational definition of multimorbidity used, including the number of diagnoses considered (primary care: 5 to all; general population: 7 to all). This last aspect seemed to be the most important factor in estimating prevalence.
  
  
  CONCLUSIONS
  Marked variation exists among studies of the prevalence of multimorbidity with respect to both methodology and findings. When undertaking such studies, investigators should carefully consider the specific diagnoses included and their number, as well as the operational definition of multimorbidity.
authors.affiliation: Department of Family Medicine, Université de Sherbrooke, Sherbrooke, Quebec, Canada. Martin.Fortin@usherbrooke.ca
authors.forename: Martin
authors.initials: M
authors.lastname: Fortin
authors.forename: Moira
authors.initials: M
authors.lastname: Stewart
authors.forename: Marie-Eve
authors.initials: ME
authors.lastname: Poitras
authors.forename: José
authors.initials: J
authors.lastname: Almirall
authors.forename: Heather
authors.initials: H
authors.lastname: Maddocks
mesh_terms: D015897:Comorbidity; D003430:Cross-Sectional Studies / Q000379:methods*; D006801:Humans; D011320:Primary Health Care