Reproducibility between two readout methods of a commercial broth microdilution assay for <i>Pseudomonas aeruginosa</i> isolates from patients with Cystic Fibrosis

<p><b>Background:</b><i>Pseudomonas aeruginosa</i> isolates from Cystic fibrosis (CF) patients are growing slowly, and frequently rendering automated susceptibility testing unsuitable. Colistin is an important antibiotic for treatment of <i>P. aeruginosa</i> infections. Broth microdilution is the only EUCAST endorsed antimicrobial susceptibility test for colistin. The VIZION™ device aids in reading broth microdilution plates and allows safe data transfer to laboratory information systems. In this study, reproducibility between visual MIC readout and readout, employing the VIZION™ device was assessed in susceptibility testing of colistin and beta-lactam antibiotics in <i>P. aeruginosa</i> isolates from CF patients.</p> <p><b>Methods:</b> Fifty-six unique <i>P. aeruginosa</i> isolates were derived from respiratory secretions of CF patients. Susceptibility testing was performed using commercially available microdilution plates. MIC readout by VIZION™ was compared to visual readout aided by a mirror (reference test).</p> <p><b>Results:</b><i>Pseudomonas aeruginosa</i> isolates displayed significantly slower growth rates compared to quality control isolates. Colistin exact MIC agreement between VIZION™ and visual readout after 24 and 48 h incubation, respectively, was 82% and 95%, essential MIC agreement was 98% and 100%, categorical agreement was 98% and 98% and reliability (weighted kappa) was 0.95 (95% CI = 0.91–0.99) and 0.99 (95% CI = 0.97–1.00). For all five antibiotics, the total number of errors (using VIZION™, in comparison with visual readout) decreased from 15 (5%) to 10 (4%) after 24 and 48 h incubation, respectively.</p> <p><b>Conclusions:</b> VIZION™ readout reproducibly determines MIC values in comparison with visual readout after 24 h of incubation. Reproducibility between the VIZION™ and visual readout increases after prolonged incubation of 48 h.</p>