Evaluation of EUCAST Direct Rapid Antimicrobial Susceptibility Test Method in Blood Culture Bottles with Positive Signal

Abstract

Early reporting of the antibiotic susceptibility testing (AST) results is essential for the survival of sepsis patients. In 2019, European Committee on Antimicrobial Susceptibility Testing (EUCAST) published a proposal to detect antimicrobial susceptibility from positive blood culture bottles with a rapid antimicrobial susceptibility test (RAST) method in a maximum of eight hours. In this study, it was aimed to evaluate the EUCAST RAST method in blood culture bottles that resulted with positive signal in BacT/ALERT (bioMerieux, France) blood culture system and that showed gram-negative bacteria with single morphology with Gram stain. The study was conducted prospectively between April 2019 and November 2019. Ninety blood culture bottles that we detected single gram negative bacteria morphology by Gram stain were tested according to the EUCAST RAST method, The isolates obtained from the blood culture bottles were studied with the EUCAST disk diffusion method and the Vitek 2 Compact (bioMerieux, France) automated system. The results obtained with RAST were compared with the results of these methods. The turn around time of the RAST method was recorded. Categorical agreement of the RAST method with conventional methods and the very major error rates were determined. Of the 14 isolates not yet covered by the EUCAST HADT method, 12 were determined to be other Enterobacterales members and two as other non-fermentatives. Two isolates were detected with the same morphological characteristics in Gram stain of the blood culture bottle and the same antibiotic susceptibility profile, but with different identification results. These sixteen isolates were excluded from the study. In this study the susceptibility of 74 isolates were determined according to the EUCAST breakpoint tables, of which 31 were Klebsiella pneumoniae, 35 were Escherichia coli, four were Acinetobacter baumannii and four were Pseudomonas aeruginosa. According to the evaluation periods of EUCAST RAST; the susceptibility profile was reported for nine (12%) of E.coli at four hours, eight (11%) at six hours, 18 (24%) at eight hours; three (4%) of K.pneumoniae at four hours, 16 (21%) at six hours, 12 (16%) at eight hours; three of P.aeruginosa (4%) at six hours, one (1%) at eight hours; two of A.baumannii (2%) at six hours and two (2%) at eight hours. The categorical aggrement of the RAST method was 91.8% with the automated system and 96.8% with the disc diffusion method. Very major errors of RAST method compared to the automated system were detected for piperacillin-tazobactam (17.7%), ceftazidime (11.6%) and meropenem (5.6%); and when compared to the disc diffusion method, for cefotaxime (5.7%) and meropenem (6.7%). Our results have shown that EUCAST RAST method can practicaly be performed in routine laboratories to report early results with a low cost. Because of the very major errors it is necessary to confirm the results with the standard methods.