Identification from Positive Blood Culture Bottles

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The page below is a sample from the LabCE course Microbial Identification Using MALDI-TOF MS. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Identification from Positive Blood Culture Bottles

Currently, there is one FDA-cleared MALDI-TOF MS-based assay for direct identification of microorganisms from positive blood cultures—the Bruker Sepsityper® Kit received FDA clearance in 2021 for the identification of more than 425 microorganisms. Research use only (RUO) applications also exist. (Always check with the manufacturer for current FDA approval status.) Because identification directly from blood offers a distinct patient care opportunity, it is predicted that more FDA-approved methods will be forthcoming. In general, organisms are concentrated from blood cultures, washed to remove debris, and processed as routine MALDI-TOF MS analysis.
Differences exist in protocols for the different MALDI-TOF MS instruments.
The Sepsityper® procedure used with the Bruker instrument (IVD-CE approved) is detailed in the figure and uses a tube-based system.
The VITEK® MS system uses a filter-vacuum approach. The blood culture specimen is placed on the filter, a vacuum separates the liquid from the cells, and reagents are added to the filter to lyse RBCs. The vacuum removes excess liquid and impurities and the sample is applied from the filter to the target for analysis.
Filter wand extraction and spotting method with reagents of the Vitek® MS Blood Culture kit.21
A complicating factor in these analyses is the presence of charcoal in blood culture media, as it has been shown to have a deleterious effect on the accuracy of identification.16,17 Some have experimented with alternative protocols that involve the filtering of blood culture solutions followed by a scraping step that obtains the microorganism for analysis.18
Despite method variation for positive blood culture analysis, numerous studies have now shown that this process is reliable for the identification of gram positive and gram negative organisms, as well as yeast. Yeast can also be identified from blood culture bottles, but the success rates achieved in the literature vary widely. Mixed cultures also pose some problems, as MALDI-TOF MS often fails to produce any identification in these circumstances. In addition, when identification is produced, only one microorganism with the most abundant mass will be identified.17,19,20
Although protocols for identifying microorganisms directly from positive blood cultures are more labor intensive than identifying microorganisms cultured on solid media, the improvement in turn-around time has been shown to significantly improve patient care. Several studies have now shown that when combined with a robust antimicrobic stewardship effort, MALDI-TOF MS can decrease hospital costs and improve outcomes.5
5. Perez KK, Olsen RJ, Musick WL, Cernoch PL, Davis JR, Peterson LE, Musser J M. "Integrating rapid diagnostics and antimicrobial stewardship improves outcomes in patients with antibiotic-resistant Gram-negative bacteremia." J Infect. 2014;69(3):216-225. Available at: http://www.journalofinfection.com/article/S0163-4453(14)00134-0/fulltext.
16. Fiori B, D'Inzeo T, Di Florio V, De Maio F, De Angelis G, Giaquinto A, Campana L, Tanzarella E, Tumbarello M, Antonelli M, Sanguinetti M, Spanu T. "Performance of two resin-containing blood culture media in detection of bloodstream infections and in direct matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) broth assays for isolate identification: clinical comparison of the BacT/Alert Plus and Bactec Plus systems." J Clin Microbiol. 2014;52(10):3558-3567. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4187791/.
17. Fothergill A, Kasinathan V, Hyman J, Walsh J, Drake T, Wang YFW. "Rapid identification of bacteria and yeasts from positive-blood-culture bottles by using a lysis-filtration method and matrix-assisted laser desorption ionization-time of flight mass spectrum analysis with the SARAMIS database." J Clin Microbio. 2013;51(3):805–809. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3592086/.
18. Machen A, Drake T, Wang YFW. "Same day identification and full panel antimicrobial susceptibility testing of bacteria from positive blood culture bottles made possible by a combined lysis-filtration method with MALDI-TOF VITEK mass spectrometry and the VITEK2 system." PloS One. 2014;9(2:e87870. Available at: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0087870.
19. Chen JHK, Ho PL, Kwan GSW, She KKK, Siu GKH, Cheng VCC, Yuen KY, Yam WC. "Direct bacterial identification in positive blood cultures by use of two commercial matrix-assisted laser desorption ionization-time of flight mass spectrometry systems." J Clin Microbiol. 2013;51(6):1733-9. Available at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3716069/.
20. Buchan BW, Riebe KM, Ledeboer NA. "Comparison of the MALDI Biotyper System Using Sepsityper Specimen Processing to Routine Microbiological Methods for Identification of Bacteria from Positive Blood Culture Bottles." J Clin Microbiol. 2012;50:346-352. Available at: http://jcm.asm.org/content/50/2/346.full.
21. Broyer, P, et al. "An Automated Sample Preparation Instrument to Accelerate Positive Blood Cultures Microbial Identification by MALDI-TOF Mass Spectrometry (Vitek® MS)." Front. Microbiol., Vol 9, 14 May 2018, https://doi.org/10.3389/fmicb.2018.00911.