Antimicrobial susceptibility testing of Haemophilus influenzae Improvement of accuracy of the disc diffusion test

Abstract Background Rapid antimicrobial susceptibility testing (RAST) of bacteria causing bloodstream infections is critical for implementation of appropriate antibiotic regimens. Objectives We have established a procedure to prepare standardized bacterial inocula for Enterobacterales-containing clinical blood cultures and assessed antimicrobial susceptibility testing (AST) data generated with the WASPLabTM automated reading system. Methods A total of 258 blood cultures containing Enterobacterales were examined. Bacteria were enumerated by flow cytometry using the UF-4000 system and adjusted to an inoculum of 106 cfu/mL. Disc diffusion plates were automatically streaked, incubated for 6, 8 and 18 h and imaged using the fully automated WASPLabTM system. Growth inhibition zones were compared with those obtained with inocula prepared from primary subcultures following the EUCAST standard method. Due to time-dependent variations of the inhibition zone diameters, early AST readings were interpreted using time-adjusted tentative breakpoints and areas of technical uncertainty. Results and conclusions Inhibition zones obtained after 18 h incubation using an inoculum of 106 cfu/mL prepared directly from blood cultures were highly concordant with those of the EUCAST standard method based on primary subcultures, with categorical agreement (CA) of 95.8%. After 6 and 8 h incubation, 89.5% and 93.0% of the isolates produced interpretable results, respectively, with CA of >98.5% and very low numbers of clinical categorization errors for both the 6 h and 8 h readings. Overall, with the standardized and automated RAST method, consistent AST data from blood cultures containing Enterobacterales can be generated after 6–8 h of incubation and subsequently confirmed by standard reading of the same plate after 18 h.

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Rapid antimicrobial susceptibility testing of positive blood cultures by direct inoculation and reading of disc diffusion tests after 3-4 hours

Apmis ◽

10.1111/apm.12897 ◽

2018 ◽

Vol 126 (11) ◽

pp. 870-876 ◽

Cited By ~ 4

Author(s):

Einar Tollaksen Weme

Keyword(s):

Antimicrobial Susceptibility ◽

Susceptibility Testing ◽

Blood Cultures ◽

Antimicrobial Susceptibility Testing ◽

Disc Diffusion ◽

Direct Inoculation

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Point-Counterpoint: Differences between the European Committee on Antimicrobial Susceptibility Testing and Clinical and Laboratory Standards Institute Recommendations for Reporting Antimicrobial Susceptibility Results

Journal of Clinical Microbiology ◽

10.1128/jcm.01129-19 ◽

2019 ◽

Vol 57 (9) ◽

Cited By ~ 16

Author(s):

Gunnar Kahlmeter ◽

Christian G. Giske ◽

Thomas J. Kirn ◽

Susan E. Sharp

Keyword(s):

Antimicrobial Susceptibility ◽

Susceptibility Testing ◽

Antimicrobial Susceptibility Testing ◽

Test Results ◽

Diffusion Test ◽

Disk Diffusion Test ◽

Technical Variability ◽

European Committee ◽

Clinical Laboratories ◽

Antibiotic Susceptibility Test

INTRODUCTION Antibiotic susceptibility test results are among the most important results issued by clinical microbiology laboratories because they routinely guide critical treatment decisions. Interpretations of MIC or disk diffusion test results, such as “susceptible” or “resistant,” are easily understood. Clinical laboratories also need to determine whether and how their reports will reflect more complex situations. Such situations include, first, whether there is need to administer higher or more frequent doses of antibiotic than usual for clinical efficacy; second, whether an antimicrobial is likely to be effective at a body site where it concentrates; and third, whether there is some uncertainty in the test results due to technical variability that cannot be eliminated. Two leading organizations that set standards for antimicrobial susceptibility testing, the European Committee on Antimicrobial Susceptibility Testing (EUCAST) and the Clinical and Laboratory Standards Institute (CLSI), have taken different strategies to deal with these challenges. In this Point-Counterpoint, Gunnar Kahlmeter and Christian Giske discuss how EUCAST is addressing these issues, and Thomas Kirn and Susan Sharp discuss the CLSI approach.

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