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Letter to the Editor

Performance of the BD Phoenix Automated Microbiology System for Trimethoprim-Sulfamethoxazole Susceptibility Testing of Staphylococcus aureus

Ghada N. Al-Rawahi, Sam Chorlton, Sukhvinder Dhaliwal, George R. Golding, Peter Tilley
Karen C. Carroll, Editor
Ghada N. Al-Rawahi
aDepartment of Pathology and Laboratory Medicine, Children’s and Women’s Health Centre of British Columbia, Vancouver, British Columbia, Canada
bDepartment of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada
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Sam Chorlton
bDepartment of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada
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Sukhvinder Dhaliwal
aDepartment of Pathology and Laboratory Medicine, Children’s and Women’s Health Centre of British Columbia, Vancouver, British Columbia, Canada
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George R. Golding
cNational Microbiology Laboratory, Public Health Agency of Canada, Winnipeg, Manitoba, Canada
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Peter Tilley
aDepartment of Pathology and Laboratory Medicine, Children’s and Women’s Health Centre of British Columbia, Vancouver, British Columbia, Canada
bDepartment of Pathology and Laboratory Medicine, University of British Columbia, Vancouver, British Columbia, Canada
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Karen C. Carroll
Johns Hopkins University School of Medicine
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DOI: 10.1128/JCM.00994-19
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LETTER

Accurate and timely antimicrobial susceptibility testing (AST) of Staphylococcus aureus is crucial for patient management. In our laboratory, S. aureus AST is performed using the BD Phoenix automated microbiology system (BD Diagnostic Systems, Sparks, MD) and cefoxitin disk diffusion (DD).

A discrepancy between trimethoprim-sulfamethoxazole (SXT) on Phoenix and SXT DD (BD BBL Sensi-Disc) was observed on a clinical methicillin-resistant S. aureus (MRSA) isolate. This discrepancy prompted us to evaluate a set of clinical isolates (6 nonrelated by spa typing MRSA isolates and 3 methicillin-susceptible S. aureus [MSSA] isolates) using Phoenix, SXT DD, Etest (bioMérieux, Marcy l’Etoile, France), and MIC test strips (MTS; Liofilchem, Italy) locally and broth microdilution (BMD) (Sensititre; Thermo Fisher Scientific) at the National Microbiology Laboratory. SXT DD and the 2 gradient diffusion tests were tested using Mueller-Hinton agar (Oxoid), and all tests were quality controlled, conducted, and interpreted according to the manufacturers’ recommendations and CLSI standards (1). All 9 isolates tested resistant by Phoenix but susceptible by DD, the 2 gradient diffusion strips, and BMD. To examine whether the observed discrepancies were related to instrument or reagent issues, another set of S. aureus isolates (n = 15) that was also SXT resistant by Phoenix but susceptible by DD was sent to Becton, Dickinson and Company for testing using 6 different lots from Phoenix panel PMIC 84 (BD catalog number 448420) and SXT DD. BD reported that 13/15 (86.7%) were resistant by Phoenix, but all 15 isolates were susceptible by SXT DD.

As a result of these findings, we prospectively added SXT DD to routine S. aureus AST. Between 25 May 2018 and 30 November 2018, 642 S. aureus isolates were tested using both methods (Table 1). Using DD as the reference method, categorical agreement was 91.9% with 50 (7.9%) major errors (MEs) and 2 (0.3%) minor errors (mEs). When we examined the MRSA subgroup (n = 70), the categorical agreement was 82.9% with 12 (17.6%) MEs and 0 mEs. There was a significantly greater number of any error type within the MRSA subgroup compared to the MSSA subgroup (odds ratio [OR], 2.7; 95% confidence interval [CI], 1.2 to 5.7; P = 0.008; Fisher’s exact test), and this difference is represented solely by MEs.

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TABLE 1

Comparison of trimethoprim-sulfamethoxazole (SXT) antimicrobial susceptibility testing of Staphylococcus aureus using the BD Phoenix automated microbiology system and disk diffusion (n = 642)a

In our experience, Phoenix achieved less than the recommended 90% categorical agreement with the reference AST method when testing MRSA against SXT (2), which is of particular concern given the limited antibiotic options for MRSA infection. The performance of the BD Phoenix automated system for SXT AST of S. aureus has been described previously (3–6). Carroll et al. (5) looked at 218 S. aureus isolates using agar dilution as the reference and found 98.6% categorical agreement and a lower rate of MEs (1.5%). Fahr et al. (6) included 114 isolates of Staphylococcus species using BMD as the reference and found 94.7% categorical agreement, lower rates of MEs (1.8%), and higher rates of very major errors (VMEs) (3.2%) and mEs (3.4%).

We hope this investigation prompts laboratories to monitor the performance of the BD Phoenix system for staphylococcal SXT susceptibility testing.

ACKNOWLEDGMENTS

We gratefully acknowledge Becton, Dickinson and Company (BD) for their assistance with testing a set of isolates with discrepant results.

  • Copyright © 2019 American Society for Microbiology.

All Rights Reserved.

REFERENCES

  1. 1.↵
    CLSI. 2019. Performance standards for antimicrobial susceptibility testing, 29th ed. CLSI supplement M100. Clinical and Laboratory Standards Institute, Wayne, PA.
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    1. Humphries RM,
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    , on behalf of the CLSI Methods Development and Standardization Working Group of the Subcommittee on Antimicrobial Susceptibility Testing. 2018. CLSI Methods Development and Standardization Working Group best practices for evaluation of antimicrobial susceptibility tests. J Clin Microbiol 56:e01934–17. doi:10.1128/JCM.01934-17.
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    . 2004. Evaluation of the automated Phoenix system for potential routine use in the clinical microbiology laboratory. J Clin Microbiol 42:1542–1546. doi:10.1128/jcm.42.4.1542-1546.2004.
    OpenUrlAbstract/FREE Full Text
  4. 4.↵
    1. Stefaniuk E,
    2. Baraniak A,
    3. Gniadkowski M,
    4. Hryniewicz W
    . 2003. Evaluation of the BD Phoenix automated identification and susceptibility testing system in clinical microbiology laboratory practice. Eur J Clin Microbiol Infect Dis 22:479–485. doi:10.1007/s10096-003-0962-y.
    OpenUrlCrossRefPubMed
  5. 5.↵
    1. Carroll KC,
    2. Borek AP,
    3. Burger C,
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    . 2006. Evaluation of the BD Phoenix automated microbiology system for identification and antimicrobial susceptibility testing of staphylococci and enterococci. J Clin Microbiol 44:2072–2077. doi:10.1128/JCM.02636-05.
    OpenUrlAbstract/FREE Full Text
  6. 6.↵
    1. Fahr A-M,
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    8. Benecchi M,
    9. Menozzi MG
    . 2003. Two-center collaborative evaluation of the performance of the BD Phoenix automated microbiology system for identification and antimicrobial susceptibility testing of Enterococcus spp. and Staphylococcus spp. J Clin Microbiol 41:1135–1142. doi:10.1128/jcm.41.3.1135-1142.2003.
    OpenUrlAbstract/FREE Full Text
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Performance of the BD Phoenix Automated Microbiology System for Trimethoprim-Sulfamethoxazole Susceptibility Testing of Staphylococcus aureus
Ghada N. Al-Rawahi, Sam Chorlton, Sukhvinder Dhaliwal, George R. Golding, Peter Tilley
Journal of Clinical Microbiology Dec 2019, 58 (1) e00994-19; DOI: 10.1128/JCM.00994-19

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Performance of the BD Phoenix Automated Microbiology System for Trimethoprim-Sulfamethoxazole Susceptibility Testing of Staphylococcus aureus
Ghada N. Al-Rawahi, Sam Chorlton, Sukhvinder Dhaliwal, George R. Golding, Peter Tilley
Journal of Clinical Microbiology Dec 2019, 58 (1) e00994-19; DOI: 10.1128/JCM.00994-19
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KEYWORDS

trimethoprim-sulfamethoxazole
Staphylococcus aureus
Phoenix

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