Direct disk diffusion test during bacteremia: evaluation of the antibiotic susceptibility results

Introduction: Bacteriemias are life-threatening emergencies. The early initiation of an adequate antibiotic therapy reduces mortality and morbidity. The aim of this work was to evaluate the results obtained with the direct AST (Antimicrobial Susceptibility Testing), carried out directly from positive blood-cultures on Mueller-Hinton CHROMagar medium (CHROMagar 4, place du 18 Juin 1940 - 75006 Paris, France) and to compare them with those obtained by the standard AST. Methods: To do this, 124 strains isolated from 124 bottles were tested against 21 antibiotics. The resulting diameters were read after 8 hours and 18 hours of incubation, interpreted using the CLSI (Clinical and Laboratory Standards Institute) breakpoints and compared to those obtained with the standard method. Results: It was found that the results were extremely satisfactory after 18 hours incubation: 94,43% CA (Categorical Agreement), 0,24% ME (Major Error) and 0,00% VME (Very Major Error), compared to less conclusive results after 8 hours of incubation (87,32% CA). The best %CA were obtained with gentamicin, sulfamethoxazole+trimethoprim, levofloxacin, ampicillin and cefoxitin at 8 hours (all >93%) and 18 hours (all >97%). Also, non-fermenting GNB recorded the best results with 98,74%CA at 18h and Staphylococcus species the lowest ones with 90,70% CA at 18h. Conclusion: The encouraging results obtained during the present study suggest a possible future implementation of the direct-from-blood culture AST as a routine technique. However, the standard AST remains the reference technique.