Direct antimicrobial susceptibility testing of Gram-negative bacteria from positive blood cultures

Every year, a great number of people die for bloodstream infections, especially by too late or inappropriate pharmacological treatment. The principal laboratory test for this type of infections is the blood culture; its limit is a turnaround time of about 96 hours. The aim of this work was to compare direct susceptibility testing with the automated VITEK 2 system. Fifty blood cultures, positive for Gram-negative bacteria, were processed for amikacin, ciprofloxacin, ceftazidime, piperacillin-tazobactam, cefotaxime, meropenem and colistin with VITEK 2 and direct susceptibility testing by Kirby-Bauer or E-test methods. We found 93.7% correlation with 5.0% minor errors, 0.5% very major errors and 0.8% major errors. Piperacillin-tazobactam has shown the lowest percentage of correlation (79%) and the highest percentage of errors (21%). To conclude, direct susceptibility testing of positive blood cultures has shown promising results, rendering this method useful for giving to clinicians preliminary informations for therapy.