Potential Misinterpretations Caused by Collapsing Upper Categories of Comorbidity Indices: An Illustration from a Cohort of Older Breast Cancer Survivors
Ahern, Thomas P.
Bosco, Jaclyn L. F.
Silliman, Rebecca A.
Yood, Marianne Ulcickas
Field, Terry S.
Lash, Timothy L.
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Citation (published version)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)
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.
Rights© 2009 Ahern et al, publisher and licensee Dove Medical Press Ltd. This is an Open Access article which permits unrestricted noncommercial use, provided the original work is properly cited.