Comparison of Rapid and Routine Methods of Identification and Antibiotic Susceptibility Testing of Microorganisms from Blood Culture Bottles

Reporting of the results of routine laboratory blood culture tests to clinicians is vital to the patients’ early treatment. This study aimed to perform identification and antibiotic susceptibility tests of the blood cultures showing positive signals of microbial growth in the first 12 hours of incubation by using centrifugation and Gram staining of 5 ml of liquid from the vial, thus achieving faster results. This study included 152 consecutively incubated blood culture samples showing positive microbial growth signals in the first 12 hours. The samples were centrifuged and then categorized into two groups (Gram-positive and Gram-negative) using Gram staining. Identification and antibiotic susceptibility tests were performed using an automated culture antibiogram device. For routine processing, media inoculated with positive blood culture were kept in the incubator for at least 24 hours. To compare the two methods in terms of the bacteria identification, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) of the growing colony was studied. By Gram staining, the same bacterial strains were obtained for 138 (92%) of the 152 samples, similar to the results of the procedures mentioned earlier. With the samples tested with both methods, the antibiotic susceptibility profiles were compared using the antibiogram results for 1,984 samples that underwent the antibiotic testing. A 97.4% (for 1,934 antibiotic susceptibility assays) agreement was observed between the two methods. Comparing the results of the post-centrifugation Gram staining to those obtained for the specimens using routine procedures, the clinicians reported a high success rate (approximately 97%).