Performance of Rapid Antibiotic Susceptibility Testing from Semiautomated Blood Culture Bottles with Disk Diffusion using CLSI Guidelines: A Cross-sectional Study

Introduction: With currently available microbiological diagnostic tools, the Turnaround Time (TAT) of a positive Blood Culture (BC) report with conventional Antimicrobial Susceptibility Testing (AST) takes 5-9 days, delaying treatment. With Rapid Antimicrobial Susceptibility Test (RAST), a positive BC can be reported within 4-8 hours. Semiautomated ColorCult BC bottles have a chemical sensor at the bottom that continuously detects the increase in carbon dioxide produced by microbial growth. As of today, the RAST method has been validated only for automated systems, not for semiautomated culture systems. Aim: To evaluate the performance of RAST from semiautomated BC bottles with disk diffusion. Materials and Methods: This cross-sectional study was conducted at Hassan Institute of Medical Sciences, Hassan, Karnataka, India from February 2024 to August 2024 and included 144 positively flagged semiautomated BC bottles (Microxpress ColorCult vial, India) showing monomicrobial growth on Gram stain. These bottles were collected and processed for both RAST and standard AST. Results obtained in RAST at 4, 6 and 8 hours were correlated with the standard AST using Clinical and Laboratory Standards Institute (CLSI) M100 as the gold standard. Equivalence criteria, according to Food and Drug Administration (FDA), were: Categorical Agreement (CA) ≥90%, very major error (vmj) ≤1.5% and major error (maj) ≤3%. Results: Among 144 positively flagged semiautomated BC bottles, 53 (36.8%) showed monomicrobial growth. Escherichia coli (n=18, 34%), Klebsiella pneumoniae (n=16, 30.2%), Acinetobacter baumannii (n=9, 17%), Pseudomonas aeruginosa (n=5, 9.4%) and Staphylococcus aureus (n=5, 9.4%) were isolated. Among all antibiotic-organism combinations tested, CA for RAST at 4-, 6- and 8-hour readings were 80.5% (252/313), 87.4% (384/439) and 91.0% (491/540), respectively. Vmj rates were 5.2% (8/152), 2.95% (7/238) and 1.64% (5/306); maj rates were 6.08% (7/115), 4.8% (7/160) and 1.65% (3/193). Among all antibiotics tested, poor CA was noted for amikacin, tobramycin, piperacillin-tazobactam and ciprofloxacin at all reading times. Conclusion: RAST performed with semiautomated BC bottles at eight hours is equivalent to standard disk diffusion using CLSI guidelines, with a marginal VME rate.