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Removal of ineligible outcome cases reduces confounding

Authors Walker AM, Schneeweiss S, DerSarkissian M, Duh MS

Received 21 December 2017

Accepted for publication 12 March 2018

Published 21 May 2018 Volume 2018:10 Pages 575—579

DOI https://doi.org/10.2147/CLEP.S160482

Checked for plagiarism Yes

Review by Single-blind

Peer reviewers approved by Dr Cristina Weinberg

Peer reviewer comments 3

Editor who approved publication: Professor Vera Ehrenstein


Video abstract presented by Alexander M Walker.

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Alexander M Walker,1 Sebastian Schneeweiss,2 Maral DerSarkissian,3 Mei Sheng Duh4

1World Health Information Science Consultants, Newton, MA, USA; 2Division of Pharmacoepidemiology, Department of Medicine, Brigham & Women’s Hospital and Harvard Medical School, Boston, MA, USA; 3Analysis Group, Inc., Los Angeles, CA, USA; 4Analysis Group, Inc., Boston, MA, USA

Background: When an exclusionary criterion is an imperfect screen, some ineligible patients will remain in a study. Medical record review for outcome adjudication can reveal such individuals.
Objective: To ascertain the circumstances under which it is advisable to remove outcome cases first found to be ineligible on chart review.
Methods: The impact on the relative risk caused by removal of ineligible outcome cases was examined under different circumstances of confounding, prevalence, and efficacy of the screening criterion for exclusions. The result is illustrated by a hospital-based cohort study in which electronic medical record diagnosis served to exclude ineligible cases, and review of text notes for putative outcome cases revealed that the codes were only 95% sensitive. Other hypothetical scenarios provide further evidence.
Results: If a condition to be excluded is a confounder of the exposure–outcome relation, residual confounding will continue to bias a study after application of an imperfect screening criterion. Removal of ineligible outcome cases after chart review creates a new bias, distinct from residual confounding. The new bias does not depend on the magnitude of the confounder–outcome association, and will be small if the exclusion criterion has resulted in a low prevalence of the exclusionary condition. The new bias caused by removal of ineligible outcome cases is almost certain to be smaller than the confounding bias that can result if they are retained.
Conclusions: Outcome cases first discovered at chart review to be study-ineligible should be removed from the study, even when similar scrutiny is infeasible for non-cases.

Keywords: exclusions, eligibility, confounding, study design, adjudication, analysis

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