Identification and Susceptibility Testing From Shortly Incubated Cultures Accelerate Blood Culture Diagnostics at No Cost.

TO THE EDITOR—We read with great interest the study by Banerjee et al [1]. This first randomized controlled trial on a “post-positivity” blood culture (BC) multiplex polymerase chain reaction (PCR) test is very much appreciated. Indeed, many recent studies on molecular-based methods that are used directly on positive BCs have been published. However, the question regarding their clinical benefit is yet to be answered. The most important concerns are high costs, additional work load, absence of uniform understanding on which samples to use, and, most important, the advent of rapid methods based on matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). The latter raises the question of whether the conventional diagnostics used for control in this study will remain a gold standard for the evaluation of new BC methods. Banerjee et al performed their study in optimal conditions, that is, they use MALDI-TOF MS for standard identification and the laboratory was operated 24 hours per day and 7 days per week (24/7). The median time to identification by standard method was 22.3 hours. We therefore conclude that in this study, MALDI-TOF MS was performed after overnight culturing. However, very short incubation on solid medium is often enough for MALDI-TOF MS identification [2]. While Banerjee et al achieved 81% identification with the PCR method, similar identification rates can be achieved with MALDI-TOF MS after only 3–5 hours of incubation [3–5]. This easy approach, which is not more complicated to perform than an automated PCR, avoids additional cost and time expenditure; one just has to observe the plates earlier and to perform MALDI-TOF MS from the “young” biomass. For example, pathogens that are not identified from mixed samples can easily be tested later from mature colonies. We and others have successfully implemented short incubation methods in routine laboratories [2– 6]; to our knowledge, this procedure is increasingly accepted. The potential of this method is even higher with 24/7 service because of subculturing of positive BCs, and timely MALDI-TOF MS from plates with sufficient growth is possible around the clock. Thus, identification is made available on the same day of BC positivity. The authors’ article title includes the phrase “susceptibility testing. ” In fact, only few resistance markers can be detected and the clinical benefit of this information is rather limited. For example, if blaKPC gene is not detected, other resistance mechanisms can still account for treatment failure with a carbapenem. On the other hand, use of cultures that had a short incubation period allows early inoculation of automated systems for aspartate aminotransferase (AST) [7] and can provide a complete final report 1 day earlier. In a 24/7 setting, real-time validation of AST results generated by automated systems would even allow diagnostics for the most positive BCs to be accomplished in less than 24 hours. All of this can be achieved by simply reorganizing laboratory workflow. For the future clinical evaluations of novel BC methods, we suggest use of the comparison with the MALDI-TOF MS identification and AST after a