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

Abstract

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.

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