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Impact of Severity of Illness Bias and Control Group Misclassification Bias in Case-Control Studies of Antimicrobial-Resistant Organisms

Published online by Cambridge University Press:  21 June 2016

Anthony D. Harris ,
Yehuda Carmeli ,
Matthew H. Samore ,
Keith S. Kaye  and
Eli Perencevich
Anthony D. Harris*
Affiliation:
Department of Epidemiology and Preventive Medicine, University of Maryland, College Park, and the Veterans Affairs Maryland Health Care System, Baltimore, Maryland
Yehuda Carmeli
Affiliation:
Tel Aviv Sourasky Medical Center, Tel Aviv, Israel
Matthew H. Samore
Affiliation:
University of Utah, Salt Lake City, Utah
Keith S. Kaye
Affiliation:
Duke University Medical Center, Durham, North Carolina
Eli Perencevich
Affiliation:
Department of Epidemiology and Preventive Medicine, University of Maryland, College Park, and the Veterans Affairs Maryland Health Care System, Baltimore, Maryland
*
Division of Healthcare Outcomes Research, Department of Epidemiology and Preventive Medicine, University of Maryland School of Medicine, 100 Greene Street, Lower Level, Baltimore, MD 21201aharris@epi.umaryland.edu
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Abstract

Background:

Case-control studies often analyze risk factors for antibiotic resistance. Recently published articles have illustrated that randomly selected control-patients may be preferable to those with the susceptible phenotype of the organism. A possible methodologic problem with randomly selected control-patients is potential bias due to control group misclassification. This occurs if some control-patients did not have clinical cultures performed and thus might have been unidentified case-patients. If this bias exists, these studies might be expected to report lower odds ratios (ORs) because control-patients would be more like case-patients.

Objective:

To analyze potential biases that might arise due to control group misclassification and potentially larger selection biases that may be introduced if control-patients are required to have at least one clinical culture.

Patients:

One hundred twenty case-patients, 770 control-patients in group 1, and 510 control-patients in group 2.

Methods:

Two case-control studies. Case-patients had clinical cultures positive for imipenem-resistant Pseudomonas aeruginosa. The first group of control-patients were random. The second group of control-patients were identical to those in group 1 except being required to have at least one clinical culture.

Results:

Univariate analyses showed higher ORs for case-patients versus control-patients in group 1 (imipenem [OR, 12.5], piperacillin-tazobactam [OR, 3.7], and vancomycin [OR, 4.7]) as compared with case-patients versus control-patients in group 2 (imipenem [OR, 8.0], piperacillin-tazobactam [OR, 2.5], and vancomycin [OR, 3.0]).

Conclusion:

Requiring control-patients to have at least one clinical culture introduces a selection bias likely because it eliminates patients with less severe illness.

Type
Orginal Articles
Information
Infection Control & Hospital Epidemiology , Volume 26 , Issue 4 , April 2005 , pp. 342 - 345
Copyright
Copyright © The Society for Healthcare Epidemiology of America 2005

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References

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