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Letter to the Editor

Some Doubts on the Study of Clinical Prognoses of Patients with a Bloodstream Infection Caused by Ampicillin-Susceptible but Penicillin-Resistant Enterococcus faecalis

Xianshi Zhou, Guanghua Tang
Xianshi Zhou
aEmergency Department, Guangdong Provincial Hospital of Chinese Medicine, Guangzhou, China
bGuangdong Provincial Key Laboratory of Research on Emergency in TCM, Guangzhou, China
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Guanghua Tang
aEmergency Department, Guangdong Provincial Hospital of Chinese Medicine, Guangzhou, China
bGuangdong Provincial Key Laboratory of Research on Emergency in TCM, Guangzhou, China
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DOI: 10.1128/AAC.01596-19
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LETTER

We read with great interest the article in an issue of Antimicrobial Agents and Chemotherapy by Kim et al. (1). The article concluded that the mortality rates of patients stratified by penicillin susceptibility might be relevant to the treatment failures of ampicillin and/or piperacillin in patients with an ampicillin-susceptible but penicillin-resistant (ASPR) Enterococcus faecalis bloodstream infection (BSI). Though an interesting and valuable study has been conducted, some methodological points need to be taken into account.

First, the values of crude effect estimates should change ≥10% after adjusting for other covariates in the multivariable model (2, 3). However, as shown in Table 1 in the article (1), for some predictors, the differences between the odds ratio (OR) values in univariate and multivariable models is far lower than the 10% cutoff. For example, the crude OR of cardiovascular disease changed only by 0.35% ([(OR crude – OR adjusted)/OR adjusted × 100%] = [(2.97 – 2.98)/2.98 × 100%] = 0.34%); thus, biases might exist in the confounder adjustment.

Second, in the study by Kim et al. (1), variables with a P value of <0.05 in the univariate analysis were analyzed whether to be included in the multivariate model. Nevertheless, only variables with P < 0.05 were imported into the multivariate model, which is questionable, as this strategy could lead to a phenomenon called testimation bias, which infers that only variables with a large effect were included and variables with a small effect did not enter into the model. This type of bias can be decreased when independent variables are imported into the multivariate model based on a P value of ≥0.2 (4, 5).

Third, Kim and colleagues applied Pearson’s correlation coefficients to describe the relationship between penicillin MIC and 30-day mortality rate. However, Pearson’s correlation should be used under the circumstance that both variables being studied are normally distributed. Thus, we suggest the authors carry out the Shapiro-Wilk test to examine whether the normality of distribution of the two variables is qualified; otherwise, the Spearman correlation should be used to investigate the correlation between two variables.

ACKNOWLEDGMENTS

Guanghua Tang was funded by Guangdong Provincial Key Laboratory of Research on Emergency in TCM (no. 2017B030314176). The work for this article was conducted at the Emergency Department, Guangdong Provincial Hospital of Chinese Medicine, Guangzhou, China. The funders had no role in writing the manuscript or the decision to submit it for publication.

We have reported that we have no relationships relevant to the contents of this paper to disclose, and we declare no competing interests. We declare that both authors participated in the conception and preparation of the manuscript.

FOOTNOTES

  • For the author reply, see https://doi.org/10.1128/AAC.01615-19.

  • Copyright © 2019 American Society for Microbiology.

All Rights Reserved.

REFERENCES

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    1. Kim D,
    2. Lee H,
    3. Yoon EJ,
    4. Hong JS,
    5. Shin JH,
    6. Uh Y,
    7. Shin KS,
    8. Shin JH,
    9. Kim YA,
    10. Park YS,
    11. Jeong SH
    . 2019. Prospective observational study of the clinical prognoses of patients with bloodstream infections caused by ampicillin-susceptible but penicillin-resistant Enterococcus faecalis. Antimicrob Agents Chemother 63. doi:10.1128/AAC.00291-19.
    OpenUrlAbstract/FREE Full Text
  2. 2.↵
    1. Maldonado G,
    2. Greenland S
    . 1993. Simulation study of confounder-selection strategies. Am J Epidemiol 138:923–936. doi:10.1093/oxfordjournals.aje.a116813.
    OpenUrlCrossRefPubMedWeb of Science
  3. 3.↵
    1. Lee PH
    . 2014. Is a cutoff of 10% appropriate for the change-in-estimate criterion of confounder identification? J Epidemiol 24:161–167. doi:10.2188/jea.je20130062.
    OpenUrlCrossRefPubMed
  4. 4.↵
    1. Mickey RM,
    2. Greenland S
    . 1989. The impact of confounder selection criteria on effect estimation. Am J Epidemiol 129:125–137. doi:10.1093/oxfordjournals.aje.a115101.
    OpenUrlCrossRefPubMedWeb of Science
  5. 5.↵
    1. Pakzad R,
    2. Safiri S
    . 2017. The effect of preoperative serum triglycerides and high-density lipoprotein-cholesterol levels on the prognosis of breast cancer: methodological issue. Breast 36:103–104. doi:10.1016/j.breast.2017.01.014.
    OpenUrlCrossRef
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Some Doubts on the Study of Clinical Prognoses of Patients with a Bloodstream Infection Caused by Ampicillin-Susceptible but Penicillin-Resistant Enterococcus faecalis
Xianshi Zhou, Guanghua Tang
Antimicrobial Agents and Chemotherapy Nov 2019, 63 (12) e01596-19; DOI: 10.1128/AAC.01596-19

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Some Doubts on the Study of Clinical Prognoses of Patients with a Bloodstream Infection Caused by Ampicillin-Susceptible but Penicillin-Resistant Enterococcus faecalis
Xianshi Zhou, Guanghua Tang
Antimicrobial Agents and Chemotherapy Nov 2019, 63 (12) e01596-19; DOI: 10.1128/AAC.01596-19
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KEYWORDS

bloodstream infection
Enterococcus faecalis
Pearson’s correlation
methodology

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