title: Multimorbidity and health-related quality of life (HRQoL) in a nationally representative population sample: implications of count versus cluster method for defining multimorbidity on HRQoL.
abstract: BACKGROUND
  No universally accepted definition of multimorbidity (MM) exists, and implications of different definitions have not been explored. This study examined the performance of the count and cluster definitions of multimorbidity on the sociodemographic profile and health-related quality of life (HRQoL) in a general population.
  
  
  METHODS
  Data were derived from the nationally representative 2007 Australian National Survey of Mental Health and Wellbeing (n = 8841). The HRQoL scores were measured using the Assessment of Quality of Life (AQoL-4D) instrument. The simple count (2+ & 3+ conditions) and hierarchical cluster methods were used to define/identify clusters of multimorbidity. Linear regression was used to assess the associations between HRQoL and multimorbidity as defined by the different methods.
  
  
  RESULTS
  The assessment of multimorbidity, which was defined using the count method, resulting in the prevalence of 26% (MM2+) and 10.1% (MM3+). Statistically significant clusters identified through hierarchical cluster analysis included heart or circulatory conditions (CVD)/arthritis (cluster-1, 9%) and major depressive disorder (MDD)/anxiety (cluster-2, 4%). A sensitivity analysis suggested that the stability of the clusters resulted from hierarchical clustering. The sociodemographic profiles were similar between MM2+, MM3+ and cluster-1, but were different from cluster-2. HRQoL was negatively associated with MM2+ (β: -0.18, SE: -0.01, p < 0.001), MM3+ (β: -0.23, SE: -0.02, p < 0.001), cluster-1 (β: -0.10, SE: 0.01, p < 0.001) and cluster-2 (β: -0.36, SE: 0.01, p < 0.001).
  
  
  CONCLUSIONS
  Our findings confirm the existence of an inverse relationship between multimorbidity and HRQoL in the Australian population and indicate that the hierarchical clustering approach is validated when the outcome of interest is HRQoL from this head-to-head comparison. Moreover, a simple count fails to identify if there are specific conditions of interest that are driving poorer HRQoL. Researchers should exercise caution when selecting a definition of multimorbidity because it may significantly influence the study outcomes.
authors.affiliation: Menzies Institute for Medical Research, University of Tasmania, 17 Liverpool Street, Hobart, TAS, 7000, Australia.
authors.forename: Lili
authors.identifier: http://orcid.org/0000-0002-2011-8626
authors.initials: L
authors.lastname: Wang
authors.affiliation: Menzies Institute for Medical Research, University of Tasmania, 17 Liverpool Street, Hobart, TAS, 7000, Australia.
authors.forename: Andrew J
authors.initials: AJ
authors.lastname: Palmer
authors.affiliation: Monash Centre for Occupation and Environmental Health (MonCOEH), Department of Epidemiology and Preventive Medicine, Monash University, Melbourne, Australia.
authors.forename: Fiona
authors.initials: F
authors.lastname: Cocker
authors.affiliation: Menzies Institute for Medical Research, University of Tasmania, 17 Liverpool Street, Hobart, TAS, 7000, Australia. Kristy.Sanderson@utas.edu.au.|School of Health Sciences, Faculty of Medicine and Health Sciences, University of East Anglia, Norwich, UK. Kristy.Sanderson@utas.edu.au.
authors.forename: Kristy
authors.initials: K
authors.lastname: Sanderson
mesh_terms: D000328:Adult; D000368:Aged; D001315:Australia / Q000453:epidemiology; D002318:Cardiovascular Diseases / Q000453:epidemiology*; D002908:Chronic Disease / Q000453:epidemiology*; D016000:Cluster Analysis; D015897:Comorbidity; D005260:Female; D006305:Health Status Indicators; D006801:Humans; D016014:Linear Models; D008297:Male; D008603:Mental Health; D008875:Middle Aged; D017063:Outcome Assessment, Health Care; D015995:Prevalence; D011788:Quality of Life / Q000523:psychology*
keywords: AQoL-4D; Definition; Health-related Quality of Life (HRQoL); Hierarchical cluster; Multimorbidity