Comparison of Antimicrobial Susceptibility Test Results of Disk Diffusion, Gradient Strip, and Automated Dilution with Broth Microdilution for Piperacillin-Tazobactam

In the treatment of septic patients, the prediction of a pathogen's susceptibility to piperacillin-tazobactam can be crucial. Commercial tests are available to measure the pathogen's susceptibility to piperacillin-tazobactam, but there is conflicting evidence regarding their accuracy. Therefore, this study compared the accuracy of disk diffusion, gradient strip, and automated dilution with the accepted standard broth microdilution. Testing was performed on 150 blood culture isolates from hospitalized patients at the University Hospital Bonn. The most recent Escherichia coli (n = 50), Klebsiella pneumoniae (n = 50), and Pseudomonas aeruginosa (n = 50) isolates were chosen. All measurements were performed strictly according to the manufacturer's instructions. Minimal inhibitory concentrations were primarily interpreted based on EUCAST (European Committee on Antimicrobial Susceptibility Testing) 8.1 and supplementarily based on CLSI (Clinical and Laboratory Standards Institute) 28th. The results of automated dilution showed a categorical agreement of 93.3% and presented five minor errors, four major errors, and one very major error. The results for gradient strip and disk diffusion were similar, except for the three additional major errors in the gradient strip and two additional very major errors in disk diffusion. Most of the major errors and very major errors were associated with P. aeruginosa. In conclusion, there was no relevant difference in accuracy between the three compared tests. Their overall categorical agreement ranged from 90.7% to 93.3% and was therefore at the lower end of the threshold. The possibly increased error rate for P. aeruginosa could be relevant in the preparation of empirical antibiotic guidelines and the treatment of septic patients.