Implementation of short incubation MALDI-TOF MS identification from positive blood cultures in routine diagnostics and effects on empiric antimicrobial therapy

The major aim of this study was to compare species identification times for microorganisms from BCs using a conventional diagnostic approach versus MALDI-TOF MS based species identification from shortly incubated biomass. Technically, the applicability, validity and microbiological accuracy of this method has been demonstrated before in a pilot study [14]. In this study, we found that the results of the pilot study held true in daily routine. Performing MALDI-TOF MS of immature cultures resulted in a significant reduction of the median time until species identification for bacteria from BCs. Particularly for fast growing Gram-negative microorganisms, the median time until species identification was reduced to less than three hours. However, compared with our pilot studies the mean time until availability of species results was slightly higher (5.9 h for Gram-positive cocci and 2 h for Gram-negative rods in the study by Idelevich et al. vs. 6.7 h for Gram-positive bacteria and 4.6 h for Gram-negative bacteria in this study) [14]. This is because in contrast to the pilot study, this data assessment includes all BC cultures even if MALDI-TOF MS was not performed on the same day (e.g. for cultures detected in the late afternoon or on weekends where the service hours of the laboratory were restricted). These results are important as they show that the pilot studies do not markedly overestimate the benefits of MALDI-TOF MS of shortly incubated cultures when this technique is applied in microbiological routine diagnostics.

The application of PCR-based BC diagnostic tests has also resulted in a significant reduction of the time until species identification [16] and, partly, results were available even faster. However, major advantages of the diagnostic approach evaluated here are that it can be done without any additional consumable costs and that it leads to cultivated microorganisms ready for further characterization, in particular susceptibility testing. The efforts regarding re-organizing workflows of technical personnel and additional hands-on time was limited in our hands and the intervention was easy to implement in diagnostic routine. However, it should be noted that this study was done in a microbiological laboratory serving a single university hospital. This limited the daily number of BC bottles, which had to be processed and facilitated communication of the results to the wards. In regional labs with larger catchment areas and service for several facilities, the implementation of this diagnostic approach might be more challenging. However, recent and future developments in the field of laboratory automation including “smart” incubators and automated growth detection may facilitate the approach reported here.

Besides the positive effects of performing MALDI-TOF MS from immature biomass on the speed of diagnostics, we also found that our lab infrastructure limits the achievements made [5, 17, 18]. As the majority of BC bottles were reported “positive” by the automated incubator outside the service hours of the laboratory, the median time until further processing of a BC bottle (i.e. microscopy and plating on solid media) was 6 h. Besides offering longer service hours, another option to solve this problem might be the installation of BC incubators in areas of the hospital available for clinical personnel so that BC bottles can be continuously loaded into the systems [19]. The second infrastructural parameter assessed in this study was the time between performing microscopy and report of this result to ward. We are convinced that the 15 min interval observed is a reasonable. Outliers (1000 min for reporting the microscopy results) were due to cases in which initial microscopy failed to identify bacteria in the Gram stain where they were actually present [20].

Besides evaluating the applicability of MALDI-TOF MS-based diagnostics in laboratory routine, we aimed to assess whether knowledge of the species (even without an antibiogram) led to adjustments of antibiotic therapies. We found that the clinicians changed empiric antibiotic therapies in 20% of all cases after communication of the species test result. This was more frequent than adjustments made based on microscopy results alone (8%). Hence, MALDI-TOF MS had an important clinical effect. On the other hand, a majority of antibiotic therapies remained unchanged. This could be explained by, first, adherence to well-designed guidelines for empiric therapies covering the reported species, second, difficulties of the treating physicians to correctly interpret the species report (without an antibiogram) with respect to how to adjust empiric therapies in a way that they are more accurate and sophisticated, or third, disregarding the diagnostic finding. Detailed evaluations of the quality of single therapeutic adjustments were difficult, as other diagnostic findings than BC results, such as allergies to antibiotics, data for organ insufficiencies and the overall prognosis of the clinical case must be considered. This is a major limitation of the retrospective study design. However, assessing the quality of adjustments after species identification in a local antibiotic stewardship team revealed that the majority of adjustments was appropriate. Clinicians particularly made adjustments for S. aureus bacteremia (32%), even if at this stage, antibiograms were not yet available. This might be due to the fact that at the UHM all patients are screened at admission for nasopharyngeal carriage of MRSA and, hence, MRSA bacteremia, which is mostly caused by strains colonizing the nares prior to infection [21], is rather unlikely, if a negative screening result from the nares is available. Moreover, the proportion of MRSA on all S. aureus isolates from BCs at our institutions followed the nationally declining trend in Germany and was 15% in 2014 (EARS-net, http://ecdc.europa.eu/, own data not shown). Interestingly, we found that in 11/13?S. aureus bacteremia cases (and 72% of all cases in which adjustments of antibiotic therapies were made) the modified therapy included the microorganism better the initial empiric regimen. Delport et al. recently reported positive clinical effects of performing MALDI-TOF MS from shortly incubated colonies. They observed in a pediatric patient collective that antibiotics were earlier optimized and the patients even had a favorable outcome and a shorter length of stay [22].