Evaluation of disc susceptibility tests performed directly from positive blood cultures

Aims: To evaluate the accuracy of direct disc susceptibility testing performed from positive BACTEC blood culture vials, using a predetermined dilution protocol. Methods: Direct susceptibility testing was performed from 432 positive blood culture vials, generating 3829 antibiotic-organism results. Results were compared with those obtained by standard disc susceptibility testing according to Clinical Laboratory Standards Institute (CLSI) methods. Results: When results were compared with the reference method, no very major errors were detected. One (0.03%) major error and 89 (2.3%) minor errors were found. Error rates by organism group ranged from 1.3% for Pseudomonas aeruginosa to 8.2% for beta-haemolytic streptococci. Conclusions: Direct susceptibility testing provided accurate susceptibility results for most organism-antibiotic combinations, with the exception of the beta-haemolytic streptococci.