Comparative Analysis of Conventional and Rapid Antimicrobial Susceptibility Testing in Patients with Gram-negative Bacteraemia Directly from Blood Culture in a Tertiary Care Hospital: A Cross-sectional Study

Introduction: Early administration of appropriate antimicrobial therapy in Gram-negative bacteraemia would influence the patient’s prognosis. Conventional Antibiotic Susceptibility Testing (AST)- disk diffusion and VITEK-2 rely on bacterial isolates obtained from subculturing positive blood cultures. Direct AST done from positive blood culture fluid would reduce subculturing time. Aim: To compare rapid AST directly from the positive blood culture with the conventional method of performing in isolated colonies obtained from positive blood cultures. Materials and Methods: The present cross-sectional study was done on 350 blood culture samples received at the diagnostic Microbiological laboratory of PSG hospital, Coimbatore, Tamil Nadu, India from August 2024 to October 2024. Consecutive positive blood cultures received during the study period showing monomicrobial (gram negative bacilli identified by doing gram stain of culture fluid) was included in the study. Blood culture samples were subjected simultaneously to susceptibility testing by Direct Sensitivity Test (DST) by Kirby bauer disk diffusion method (CLSI recommended) and Antibiotic Sensitivity Test (AST) by Vitek-2 Compact which is an automated (BioMerieux) reference method from positive blood cultures flagged by BacT/ALER3D System. AST was done directly from the positive blood culture fluids and on sub cultures by disc diffusion and VITEK 2 method. Data analysed using Statistical Package for Social Sciences (SPSS) v.28.0 and p-value less than 0.05 was considered statistically significant. Results: Antibiotic susceptibility test results of Klebsiella pneumoniae and E.Coli between direct and conventional disc diffusion method showed complete agreement in 98% of the samples in total. More than 95% samples showed complete agreement for all the antibiotics. On comparison between direct and conventional VITEK 2 method, 99% of the samples showed complete agreement between two tests in total. A 100% of samples showed complete agreement for the antibiotics like ceftazidime, ceftazidime+clavulanic acid, cefoxitin, cotrimoxazole, meropenem, ertapenem, piperacillin-tazobactam, cefaperazonesulbactam, colistin. 91.25 and 90.7% samples showed complete agreement for amikacin and Ciprofloxacin. Conclusion: The therapeutic value of this approach is underlined by the excellent agreement rates obtained for antibiotics of critical importance. This may contribute to improved outcomes through earlier directed therapy.