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Clin Epidemiol. 2009 Aug 9;1:93-100.

Potential misinterpretations caused by collapsing upper categories of comorbidity indices: An illustration from a cohort of older breast cancer survivors.

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1
Department of Epidemiology, Boston University School of Public Health, Boston, MA, USA;

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.

KEYWORDS:

bias (epidemiologic); breast neoplasms; comorbidity; confounding factors (epidemiologic); epidemiology; statistical models

PMID:
20865090
PMCID:
PMC2943165
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