Bloodstream infection (BSI) is one of the leading causes of death around the world, with an estimated incidence of up to 19 million people worldwide every year.1 Rapid identification of the causative organism is essential to guide clinicians in the selection of the most appropriate targeted treatment for patients with BSI, and it is associated with improved patient outcomes.2 Many remarkable improvements have been made in the attempt to reduce the time required to identify the pathogen in positive blood cultures, including the direct inoculation of fluids from positive blood cultures into automated systems, fluorescent in situ hybridization (FISH), and PCR. All these methods, however, are expensive and require several hours of work.3

In recent years, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) has emerged as the new standard in bacterial identification and is now being adopted in clinical laboratories worldwide.

MALDI-TOF technology uses a laser to softly ionize the many structural elements (primarily ribosomal proteins) of bacteria and yeasts and then separate the molecules according to their mass-to-charge ratio (m/z).4,5 Four commercial systems are currently in use worldwide: the MALDI-TOF BioTyper (Bruker Daltonics, Bremen, Germany); Saramis (AnagnosTec, Potsdam, Germany); Andromas (Andromas, Paris, France), and Vitek MS (bioMérieux, Marcy l’Étoile, France), although Bruker's BioTyper system is the most widely used in clinical practice and research.

Different studies have reported successful identification of bacteria directly from positive blood cultures using MALDI-TOF MS. Nevertheless, the results of these studies have varied, depending on the distribution of microbial isolates, the pre-treatment/extraction method applied, and where the log score cut-off values were defined.6 One of these methods has become commercially available as the Sepsityper Kit (Bruker Daltonics) to standardize the preparation of blood culture prior to spectrometric analysis. The method involves lysis of blood cells, followed by centrifugation and washing steps. The final result is a pellet of bacteria, which is further processed by standard methods for identification using MALDI-TOF.7,8

One disadvantage of the Sepsityper kit is the additional cost, so that many laboratories have developed their own in-house methods of bacterial extraction from positive blood culture bottles.9–11 These methods vary in their approach to removing human cellular components and enriching the microbes from blood culture fluids. Saponin12 and ammonium chloride13 to lyse the blood cells have been described. Separation of microorganisms from blood cells can be performed with differential centrifugation and gel separator tubes.14,15 Simple stepwise sedimentation of blood cells and microorganisms has also been described.5,16,17

The present systematic review and meta-analysis aimed to evaluate the accuracy of MALDI-TOF for the identification of bacteria directly from positive blood culture bottles. We analyzed the different pre-treatment methods and other variables that may have influenced the correct identification of bacteria to species level from positive blood culture bottles.

We designed a systematic review and meta-analysis of the scientific literature. Our study was performed according to the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines for systematic reviews.18

This meta-analysis synthesizes the available evidence about the validity of the MALDI-TOF system for the accurate identification of bacteria directly from positive blood cultures. An assessment of the results suggests that MALDI-TOF provides highly accurate identification of Gram-negative bacteria at the species level directly from positive blood cultures. For Gram-positive bacteria, overall accuracy is moderate. Individual studies have previously evaluated the different percentages of correct identifications between Gram-negative and Gram-positive bacteria.12,41 This has several potential clinical implications. On the one hand, because the susceptibility patterns of some Gram-negative bacteria (e.g., P. aeruginosa, A. baumannii, some Enterobacteriaceae) are typically different, accurate identification is important and could therefore allow more rapid optimization of targeted therapy. On the other hand, the results found for Gram-positive are somewhat worrisome; the early management of BSI due to S. aureus and coagulase-negative staphylococci can be quite different, so that misidentification of these bacteria and other Gram-positive may have negative consequences for patient care.

It is not yet understood why it is not always possible to identify Gram-positive bacteria directly from positive blood cultures. It has been speculated that the more robust cell wall decreases protein extraction efficacy and that the slow growth of some species can lead to a very small pellet after the extraction. Several studies have shown that, in terms of efficacy (percentage of correct identifications compared to established reference methods), the Sepsityper performs best with Gram-negative bacteria. Nevertheless, even with Gram-positive bacteria, some researchers report successful identifications in around 75% or more of positive blood cultures.48

Various studies have been published comparing the results obtained with the Sepsityper system and different in-house methods before carrying out the MALDI-TOF analysis. However, while some studies have demonstrated that alternative extraction methods may yield results identical to or better than the Sepsityper kit,6,10,41 others have demonstrated the superiority of the Sepsityper extraction method over in-house methods.16,46 To determine the possible advantage of the Sepsityper kit for the correct identification of Gram-positive bacteria, we analyzed separately those studies that had used in-house methods for pretreatment of the sample and those that had used the commercial Sepsityper method. Although the Sepsityper method showed higher identification rates for some species, the in-house methods generally generated similar results to those obtained with Sepsityper.

Overall, our results showed no major differences in the identification of Gram-positive bacteria when the Sepsityper method or in-house methods were used, although the heterogeneity of results arising from studies that used the Sepsityper method was lower than when in-house methods were used. Nevertheless, the rate of correct identifications of S. aureus was clearly higher with the Sepsityper method, and S. pneumoniae showed superior results with the in-house, as against the Sepsityper method. The results for other Staphylococcus spp. and Streptococcus spp. were similar, with no differences between the two methods. For the identification of Enterococcus spp., the results recorded were slightly higher using the Sepsityper, compared to in-house methods. As with previous reports, our results suggest that the decision to implement one or the other depends on the individual laboratory.

In most cases, the methodological quality of the studies included in this review is moderate. The studies have various limitations as well as methodological issues, with problems of both internal and external validity. Some of these limitations were: the selection of samples included in the different studies; lack of blinding; both tests in the included studies were not realized simultaneously; the intervention differed depending on whether the method used was in-house or commercial; likewise, the subsequent interpretation of the data varied as a result of the different cut-offs used; finally the microbiological reference method in the included studies was performed differently, using different techniques and procedures.

To the best of our knowledge, this is the first meta-analysis of studies of diagnostic tests to assess the accuracy of MALDI-TOF for the rapid, direct identification of bacteria from positive blood cultures. Earlier studies have evaluated the MALDI-TOF system for the identification of clinical bacteria; Drancourt49 published a review of MALDI-TOF for the detection of microorganisms in the blood; Morgenthaler et al.48 reviewed only the standard Sepsityper method for rapid identification of microorganisms, including in-house methods, although the review was not systematic. In 2015, Dixon et al.50 carried out a systematic review comparing MALDI-TOF and conventional identification methods for the rapid identification of pathogens in patients with suspected or known BSI, and also collected information on reduced hospital costs, length of stay, and ‘time to appropriate antimicrobial treatment’. Ling et al.,51 in 2014, published a meta-analysis of the accuracy of MALDI-TOF for the identification of clinical pathogenic fungi, but not bacteria.

This meta-analysis has a number of limitations. First, the number and heterogeneity of the studies posed a variety of research questions. A second limitation was potential publication bias; we excluded unpublished papers, gray literature and industry reports. We tried to avoid this bias as much as possible by using several databases and performing searches without language restrictions. In order to avoid possible bias in the application of selection criteria, these were pre-specified a priori. A third limitation is that some studies conducted the test with two or three different interventions (e.g.: Sepsityper and one or two in-house methods). We assumed that each intervention was a new independent study, so that the total number of evaluated strains was higher. A fourth limitation is that some studies presented score cut-off values higher than 2.0 for the MALDI-TOF detection system. We assumed that all results had a cutoff point of ≥2.0 to try and ensure standardization of the results. Another possible limitation was that, although all studies used an adequate gold standard as comparator, these varied across the studies, so that the results would have varied depending on the quality of the gold standard. The results of this study therefore should be interpreted with caution due to these potential limitations. With respect to the high heterogeneity obtained for overall Gram-negative bacteria in the meta-analysis, this may be related to the inclusion of different species where the identification accuracy of the MALDI-TOF system was heterogeneous, since heterogeneity decreased substantially when the accuracy of each specific species of Gram-negative bacteria was analyzed separately, as previously stated. On the other hand, the main reason for high heterogeneity among Gram-positive bacteria could be the inclusion of the two methods (both in-house or commercial) in the same overall meta-analysis, since it was previously confirmed in our separate analyses that heterogeneity for each Gram-positive species was higher with in-house methods. Another possible limitation of our work has been that from the end of it until its possible publication, new articles of interest have been published in the scientific literature, which could partially modify the results of our systematic review.

Finally, this meta-analysis highlights the need to perform new diagnostic studies to assess the accuracy of MALDI-TOF for Gram-positive bacteria with standardized methods, using the same comparator to improve the methodology. A faster time-to-result is an advantage for the patient, since, in combination with antibiotic stewardship programs, optimized antibiotic therapy can very often be administered on the basis of a species/genus identification of the underlying microorganisms. Looking to the future, we may suppose that this technology can be used in the detection of bacterial resistance determinants and virulence factors.

In summary, MALDI-TOF MS demonstrated high accuracy for the direct identification of Gram-negative bacteria from blood culture and moderate accuracy for Gram-positive bacteria (low in some species). There were no major differences with respect to the identification of Gram-positive bacteria when either in-house methods or the standard Sepsityper kit were used.

All authors confirm that there is no conflict of interest for the publication of this work.