title: Potential misinterpretations caused by collapsing upper categories of comorbidity indices: An illustration from a cohort of older breast cancer survivors.
abstract: BACKGROUND
  Comorbidity indices summarize complex medical histories into concise ordinal scales, facilitating stratification and regression in epidemiologic analyses. Low subject prevalence in the highest strata of a comorbidity index often prompts combination of upper categories into a single stratum ('collapsing').
  
  
  OBJECTIVE
  We use data from a breast cancer cohort to illustrate potential inferential errors resulting from collapsing a comorbidity index.
  
  
  METHODS
  Starting from a full index (0, 1, 2, 3, and ≥4 comorbidities), we sequentially collapsed upper categories to yield three collapsed categorizations. The full and collapsed categorizations were applied to analyses of (1) the association between comorbidity and all-cause mortality, wherein comorbidity was the exposure; (2) the association between older age and all-cause mortality, wherein comorbidity was a candidate confounder or effect modifier.
  
  
  RESULTS
  COLLAPSING THE INDEX ATTENUATED THE ASSOCIATION BETWEEN COMORBIDITY AND MORTALITY (RISK RATIO, FULL VERSUS DICHOTOMIZED CATEGORIZATION: 4.6 vs 2.1), reduced the apparent magnitude of confounding by comorbidity of the age/mortality association (relative risk due to confounding, full versus dichotomized categorization: 1.14 vs 1.09), and obscured modification of the association between age and mortality on both the absolute and relative scales.
  
  
  CONCLUSIONS
  Collapsing categories of a comorbidity index can alter inferences concerning comorbidity as an exposure, confounder and effect modifier.
authors.affiliation: Department of Epidemiology, Boston University School of Public Health, Boston, MA, USA;
authors.forename: Thomas P
authors.initials: TP
authors.lastname: Ahern
authors.forename: Jaclyn Lf
authors.initials: JL
authors.lastname: Bosco
authors.forename: Rebecca A
authors.initials: RA
authors.lastname: Silliman
authors.forename: Marianne Ulcickas
authors.initials: MU
authors.lastname: Yood
authors.forename: Terry S
authors.initials: TS
authors.lastname: Field
authors.forename: Feifei
authors.initials: F
authors.lastname: Wei
authors.forename: Timothy L
authors.initials: TL
authors.lastname: Lash