Letters to the Editor Epidemiology and risk factors for Clostridium difficile-associated diarrhea in adult inpatients in a university hospital in China: Methodological issues To the Editor: We read with interest the article recently published in the Amer- ican Journal of Infection Control by Tang and colleagues. 1 The authors investigated the risk factors for Clostridium difficile-associated di- arrhea at a university hospital in eastern China. 1 Although the study reported several interesting findings, several methodological issues must be considered. First, the article does not mention which method was used to select the predictors included in the multivariable analysis. As shown in Table 2, 1 the authors included only length of hospital stay (≥6 days), comorbidity (e.g., diabetes), and treatment type (e.g., coloclysis and proton-pump inhibitor) in the multivariable analysis. This is puz- zling, as their univariable analysis showed statistically significant (P < .05) associations for other predictors, such as the use of differ- ent types of antibiotics (e.g., cephalosporin and fluoroquinolones). Investigators normally conduct multivariable analyses using a step- wise method, selecting the variables to retain using statistical criteria, or they select predictors according to their clinical relevance. 2 However, the approach used by Tang and colleagues 1 is not clear. Second, the differences in odds ratios between the univariable and multivariable models for some predictors (e.g. comorbidity with diabetes) were relatively small. Generally, it has been reported that the unadjusted exposure-outcome should be changed by a certain percentage (e.g., 10%) in the multivariable analysis. When this is not the case, it is likely to be caused by the degree of residual confounding. 3 Acknowledgment This work was not supported by any organization. References 1. Tang C, Li Y, Liu C, Sun P, Huang X, Xia W, et al. Epidemiology and risk factors for Clostridium difficile-associated diarrhea in adult inpatients in a university hospital in China. Am J Infect Control 2018;46:285-90. 2. Steyerberg EW. Clinical prediction models: a practical approach to development, validation, and updating. Springer Science & Business Media; 2008. 3. Lee PH. Is a cutoff of 10% appropriate for the change-in-estimate criterion of confounder identification? J Epidemiol 2014;24:161-7. Conflict of interest: None to report. Saeid Safiri, MSc, PhD* Managerial Epidemiology Research Center, Department of Public Health, School of Nursing and Midwifery, Maragheh University of Medical Sciences, Maragheh, Iran Department of Epidemiology & Biostatistics, School of Public Health, Tehran University of Medical Sciences, Tehran, Iran Mark J.M. Sullman, PhD Middle East Technical University, Northern Cyprus Campus, Güzelyurt/Morphou, Northern Cyprus * Address correspondence to Saeid Safiri, MSc, PhD, Managerial Epidemiology Research Center, Department of Public Health, School of Nursing and Midwifery, Maragheh University of Medical Sciences, Maragheh, Iran. E-mail address: saeidsafiri@gmail.com (S. Safiri). https://doi.org/10.1016/j.ajic.2018.01.022 Regarding “Epidemiology and risk factors for Clostridium difficile- associated diarrhea in adult inpatients in a university hospital in China: Methodologic issues” To the Editor: We appreciate the interest in our study and would like to respond to the methodology-related issues raised by Safiri and Sullman in their letter to Editor. The letter concerned 2 statistics questions, mainly variable selection in the multivariable analysis and con- founder identification. VARIABLE SELECTION IN THE MULTIVARIABLE ANALYSIS The forward selection method was used in our study and we apologize if not mentioning that in our article made Safiri and Sullman confused. We did mention that “some well- acknowledged factors had no significant difference in the study, such as age >64 years and nasogastric tube feeding, but they were also included for multivariate logistic regression alone 0196-6553/© 2018 Association for Professionals in Infection Control and Epidemiology, Inc. Published by Elsevier Inc. All rights reserved. American Journal of Infection Control 46 (2018) 597-8 Contents lists available at ScienceDirect American Journal of Infection Control journal homepage: www.ajicjournal.org American Journal of Infection Control