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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 are no FDA cleared MALDI-TOF MS based assays for direct identification of microorganisms from positive blood cultures. Because identification directly from blood offers a distinct patient care opportunity, it is predicted that FDA approved methods will be soon to arrive. In general, organisms are concentrated from blood cultures, washed to remove debris, and processed as for routine MALDI-TOF MS analysis
Differences exist in protocols for the different MALDI-TOF MS instrument.
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.
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 (Fothergill et al. 2013, Flori et al. 2014). 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 (Machen et al. 2014).
Despite method variation, numerous studies have now shown that this process is reliable for 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 (Chen et al. 2013, Fothergill et al. 2013, Buchan et al. 2012).
Although protocols for identifying microorganism 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 (Perez et al. 2014).