Potential misinterpretations caused by collapsing upper categories of comorbidity indices: An illustration from a cohort of older breast cancer survivors
Thomas P Ahern1, Jaclyn LF Bosco2, Rebecca A Silliman2, Marianne Ulcickas Yood3, Terry S Field4, Feifei Wei5, Timothy L Lash1, On behalf of the BOW Investigators
1Department of Epidemiology, Boston University School of Public Health, Boston, MA, USA; 2Department of Medicine, Section of Geriatrics, Boston University School of Medicine, Boston, MA, USA; 3Department of Epidemiology and Public Health, Yale University School of Medicine, New Haven, CT, USA; 4Meyers Primary Care Institute, University of Massachusetts Medical School, Worcester, MA, USA; 5HealthPartners Research Foundation, Minneapolis, MN, USA
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: epidemiology, breast neoplasms, comorbidity, confounding factors (epidemiologic), bias (epidemiologic), statistical models
This work is published by Dove Medical Press Limited, and licensed under Creative Commons Attribution - Non Commercial (unported, v3.0) License. The full terms of the License are available at http://creativecommons.org/licenses/by-nc/3.0/. Non-commercial uses of the work are permitted without any further permission from Dove Medical Press Limited, provided the work is properly attributed. Permissions beyond the scope of the License are administered by Dove Medical Press Limited. Information on how to request permission may be found at: http://www.dovepress.com/permissions.php
Other article by this author:
Concordance of metabolic enzyme genotypes assayed from paraffin-embedded, formalin-fixed breast tumors and normal lymphatic tissue
Thomas P Ahern, Mariann Christensen, Deirdre P Cronin-Fenton, et al
Published Date: 22 October 2010
Readers of this article also read:
Prozorova GF, Pozdnyakov AS, Kuznetsova NP, Korzhova SA, Emel’yanov AI, Ermakova TG, Fadeeva TV, Sosedova LM
Published Date: 16 April 2014
Patterns of 6-mercaptopurine and azathioprine maintenance therapy among a cohort of commercially insured individuals diagnosed with Crohn's disease in the United States
Lund JL, Cook SF, Allen JK, Carroll CF, Kappelman MD
Published Date: 6 December 2013
Ashwanikumar N, Kumar NA, Nair SA, Kumar GS
Published Date: 15 November 2012
A novel preparation method for silicone oil nanoemulsions and its application for coating hair with silicone
Hu Z, Liao M, Chen Y, Cai Y, Meng L, Liu Y, Lv N, Liu Z, Yuan W
Published Date: 12 November 2012
Deepa G, Thulasidasan AK, Anto RJ, Pillai JJ, Kumar GS
Published Date: 27 July 2012
Chen ZQ, Liu Y, Zhao JH, Wang L, Feng NP
Published Date: 30 March 2012
Published Date: 16 February 2012
Particle size reduction to the nanometer range: a promising approach to improve buccal absorption of poorly water-soluble drugs
Rao S, Song Y, Peddie F, Evans AM
Published Date: 20 June 2011
Intracellular heavy metal nanoparticle storage: progressive accumulation within lymph nodes with transformation from chronic inflammation to malignancy
Tommaso Iannitti, Stefania Capone, Antonietta Gatti, et al
Published Date: 15 November 2010
Pitipol Choopong, Nattaporn Tesavibul, Nattawut Rodanant
Published Date: 14 July 2010