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Title: Potential Misinterpretations Caused by Collapsing Upper Categories of Comorbidity Indices: An Illustration from a Cohort of Older Breast Cancer Survivors
Authors: Ahern, Thomas P;Bosco, Jaclyn LF;Silliman, Rebecca A;Yood, Marianne Ulcickas;Field, Terry S;Wei, Feifei;Lash, Timothy L
subject: Epidemiology;Breast neoplasms;Comorbidity;Confounding factors (epidemiology);Bias (epidemiology);Statistical models
Year: 9-Aug-2009
Publisher: Dove Medical Press
Description: 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.
National Cancer Institute (R01 CA 093772, R01 CA 118708, K05 CA 092395); Congressional Directed Medical Research Programs pre-doctoral training award (BC073012)
Standard no: Ahern, Thomas P, Jaclyn LF Bosco, Rebecca A Silliman, Marianne Ulcickas Yood, Terry S Field, Feifei Wei, Timothy L Lash. "Potential misinterpretations caused by collapsing upper categories of comorbidity indices: An illustration from a cohort of older breast cancer survivors" Clinical Epidemiology 1:93-100. (2009)
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