Back to Browse Journals » Clinical Epidemiology » Volume 1

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

Authors Thomas P Ahern, Jaclyn LF Bosco, Rebecca A Silliman, Marianne Ulcickas Yood, et al.

Published Date June 2009 Volume 2009:1 Pages 93—100

DOI http://dx.doi.org/10.2147/CLEP.S5757

Published 23 June 2009

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

Download Article [PDF] 

Creative Commons License 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

Clinical Epidemiology 2010, 2:241-246

Published Date: 22 October 2010

Readers of this article also read:

MRI-detectable polymeric micelles incorporating platinum anticancer drugs enhance survival in an advanced hepatocellular carcinoma model

Vinh NQ, Naka S, Cabral H, Murayama H, Kaida S, Kataoka K, Morikawa S, Tani T

International Journal of Nanomedicine 2015, 10:4137-4147

Published Date: 25 June 2015

Peak expiratory flow rate as a surrogate for forced expiratory volume in 1 second in COPD severity classification in Thailand

Pothirat C, Chaiwong W, Phetsuk N, Liwsrisakun C, Bumroongkit C, Deesomchok A, Theerakittikul T, Limsukon A

International Journal of Chronic Obstructive Pulmonary Disease 2015, 10:1213-1218

Published Date: 25 June 2015

Sustained release of VH and rhBMP-2 from nanoporous magnesium–zinc–silicon xerogels for osteomyelitis treatment and bone repair

Li FQ, Wu W, Xiang L, Weng G, Hong H, Jiang H, Qian J

International Journal of Nanomedicine 2015, 10:4071-4080

Published Date: 22 June 2015

Sustained-release nanoART formulation for the treatment of neuroAIDS

Jayant RD, Atluri VSR, Agudelo M, Sagar V, Kaushik A, Nair M

International Journal of Nanomedicine 2015, 10:1077-1093

Published Date: 4 February 2015

Lower irritation microemulsion-based rotigotine gel: formulation optimization and in vitro and in vivo studies

Wang Z, Mu HJ, Zhang XM, Ma PK, Lian SN, Zhang FP, Chu SY, Zhang WW, Wang AP, Wang WY, Sun KX

International Journal of Nanomedicine 2015, 10:633-644

Published Date: 14 January 2015

Perioperative hyperglycemia is associated with postoperative neurocognitive disorders after cardiac surgery

Zhang X, Yan X, Gorman J, Hoffman SN, Zhang L, Boscarino JA

Neuropsychiatric Disease and Treatment 2014, 10:361-370

Published Date: 19 February 2014

Methacrylic-based nanogels for the pH-sensitive delivery of 5-Fluorouracil in the colon

Ashwanikumar N, Kumar NA, Nair SA, Kumar GS

International Journal of Nanomedicine 2012, 7:5769-5779

Published Date: 15 November 2012

Development of a nanoporous and multilayer drug-delivery platform for medical implants

Karagkiozaki V, Vavoulidis E, Karagiannidis PG, Gioti M, Fatouros DG, Vizirianakis IS, Logothetidis S

International Journal of Nanomedicine 2012, 7:5327-5338

Published Date: 8 October 2012

Concentration- and time-dependent response of human gingival fibroblasts to fibroblast growth factor 2 immobilized on titanium dental implants

Ma Q, Wang W, Chu PK, Mei S, Ji K, Jin L, Zhang Y,

International Journal of Nanomedicine 2012, 7:1965-1976

Published Date: 17 April 2012

Functionalization of single-walled carbon nanotubes and their binding to cancer cells

Madani SY, Tan A, Dwek M, Seifalian AM

International Journal of Nanomedicine 2012, 7:905-914

Published Date: 22 February 2012