Evaluation of Rapid antimicrobial susceptibility testing (RAST) by disk diffusion on early subculture growth from positive blood culture bottles.

Antimicrobial susceptibility testing (AST) of bacteria isolated in blood cultures is critical for management of patients with sepsis. Standard turnaround time (TAT) to isolate and perform AST is around 48-96 h. This study was planned to evaluate the performance of rapid antimicrobial susceptibility testing (RAST) on early subculture (s/c) growth of positive blood culture bottle (PBCB) and compare this with standard 24 h (h) growth. Gram stain along with s/c was done for positive blood culture bottle (PBCB) in three sets of plates that were incubated at 370C for 4,8 and 24 h. Each plate was examined for growth after 4, 8 and 24 h and AST was performed from each plate and was read after 4, 8 and 24 h for each plate. Out of 161 isolates, 84.4 % (136/161) isolates were gram-negative bacilli (GNB) and15.5 % (25/161) were identified as gram-positive cocci (GPC). Our study results showed that interpretable RAST results for Escherichia coli,Klebsiella pneumoniae,Acinetobacter species, Serratia marcescens and Enterobacter species could be achieved for 4 h s/c growth whose AST is read at 4 h. For Pseudomonas aeruginosa, Burkholderia cepacia complex and Stenotrophomonas maltophilia interpretable RAST results could be achieved for 8 h s/c growth whose AST is read at 4 h. For Staphylococcus species, Enterococcus species, Ochrobactrum, Achromobacter and Ralstonia species interpretable RAST results could be achieved for 24 h s/c growth whose AST is read at 4 h. RAST testing from the early s/c growth can be performed to decrease the TAT of AST results.