False susceptibility to cefotetan reported by MicroScan for DHA-type AmpC beta-lactamase-producing Klebsiella...

Area of Science:

• Clinical Microbiology
• Antimicrobial Resistance
• Bacterial Genetics

Background:

• AmpC-producing Klebsiella pneumoniae is a growing concern in healthcare settings.
• Accurate susceptibility testing is crucial for effective antibiotic treatment.
• Cefotetan is a beta-lactam antibiotic used to treat infections caused by Gram-negative bacteria.

Purpose of the Study:

• To evaluate the diagnostic accuracy of the MicroScan WalkAway system for cefotetan susceptibility testing.
• To assess the performance of the MicroScan system against a reference method for AmpC-producing K. pneumoniae isolates.

Main Methods:

• Fifty-seven Klebsiella pneumoniae isolates with AmpC production (indicated by D-shape flattening in double-disk synergy test) were analyzed.
• Cefotetan Minimum Inhibitory Concentrations (MICs) were determined using the gold standard agar dilution method.
• The bla(DHA) and bla(CMY-1) genes were identified via molecular methods.

Main Results:

• The MicroScan WalkAway system classified isolates as susceptible (n=28), intermediate (n=18), or resistant (n=11).
• Comparison with agar dilution revealed high error rates: very major errors (28.1%), minor errors (47.4%), and major errors (1.8%).
• All isolates harbored the bla(DHA) gene, with one also carrying bla(CMY-1).

Conclusions:

• The MicroScan WalkAway system demonstrates significant limitations in accurately determining cefotetan susceptibility for AmpC-producing K. pneumoniae.
• The high rates of very major and minor errors suggest a potential for misinterpretation of susceptibility results.
• Clinical laboratories should exercise caution when relying solely on the MicroScan system for cefotetan susceptibility testing in these specific isolates.