Automated direct screening for resistance of Gram-negative blood cultures using the BD Kiestra WorkCell

Early detection of resistance in sepsis due to Gram-negative organisms may lead to improved outcomes by reducing the time to effective antibiotic therapy. Traditional methods of resistance detection require incubation times of 18 to 48 h to detect resistance. We have utilised automated specimen processing, digital imaging and zone size measurements in conjunction with direct disc susceptibility testing to develop a method for the rapid screening of Gram-negative blood culture isolates for resistance. Positive clinical blood cultures with Gram-negative organisms were prospectively identified and additional resistant mock specimens were prepared. Broth was plated and antibiotic-impregnated discs (ampicillin, ceftriaxone, piperacillin-tazobactam, meropenem, ciprofloxacin, gentamicin) were added. Plates were incubated, digitally imaged and zone sizes were measured using the BD Kiestra WorkCell laboratory automation system. Minimum, clinically useful, incubation times and optimised zone size cut-offs for resistance detection were determined. We included 187 blood cultures in the study. At 5 h of incubation, > 90% of plates yielded interpretable results. Using optimised zone size cut-offs, the sensitivity for resistance detection ranged from 87 to 100%, while the specificity ranged from 84.7 to 100%. The sensitivity and specificity for piperacillin-tazobactam resistance detection was consistently worse than for the other agents. Automated direct disc susceptibility screening is a rapid and sensitive tool for resistance detection in Gram-negative isolates from blood cultures for most of the agents tested.