Rapid antimicrobial susceptibility testing of positive blood cultures by direct inoculation and reading of disc diffusion tests after 3-4 hours.

The aim of the study was to validate whether rapid antimicrobial susceptibility testing (AST) by very early reading of disc diffusion tests would provide reliable results in daily routine work. A total of 264 positive blood culture bottles were examined, of which 178 were examined as part of the daily routine workflow. Enterobacterales were evaluated for resistance to cefotaxime, ceftazidime, gentamicin and ampicillin. Staphylococcus aureus (S. aureus) was tested for resistance to cefoxitin as a marker of methicillin-resistant S. aureus (MRSA). The zones were read after 3 h, and if there was insufficient growth, 30 and 60 min later. The results were compared to standard overnight AST. For ampicillin, gentamicin and cefoxitin, there were no errors. For cefotaxime, there were three minor (1.5%), three major (1.5%) and no very major errors. For ceftazidime, there were 13 minor (6.5%) errors only. With the exception of one minor error, all errors were ESBL-A- or AmpC-producing isolates where rapid AST showed a higher degree of resistance than standard AST. This low-cost method may contribute to early effective antibacterial treatment by providing reliable results of AST within 3-4 h. Special breakpoints for early reading are a prerequisite.