Comparison of different processing methods of blood cultures – impact of MALDI-TOF MS and direct antimicrobial susceptibility testing (AST) on accuracy and time to result

Abstract Introduction: Our laboratory uses MALDI-TOF MS (MALDI) and the VITEK®2 system (DV2) directly from positive blood cultures (BC) for organism identification (ID) and antimicrobial susceptibility testing (AST). Our objective was to compare direct MALDI/DV2 to a commercial BC ID/AST platform, the Accelerate Pheno™ system, in the ID/AST of clinical and seeded BC positive for Gram-negative bacilli (GNB). Methods: BC positive for GNB were collected over a three month period and tested using AXDX, direct MALDI/DV2, and compared to conventional methods. A subset of sterile BC were seeded with multi-drug resistant GNB. Discrepancies in very major errors (VME) and major errors (ME) were confirmed with broth microdilution. Results: A total of 29 clinical samples and 35 seeded samples were analyzed. Direct MALDI had a higher ID failure rate (31.0%) compared to AXDX (3.4%) [p<0.001]. Time to ID/AST was 1.5/6.9 hours, 5.8/16.5 hours and 21.6/33.0 hours for AXDX, direct MALDI/DV2 and conventional methods, respectively (p<0.001). For clinical samples, AXDX and DV2 had essential agreement (EA)/categorical agreement (CA) >96%. For seeded samples, AXDX had EA, CA, VME, ME and mE of 93.2%, 89.0%, 2.2%, 0%, and 9.2%, respectively. AXDX had a large number of non-reports (6.1%), stemming from meropenem testing. DV2 had EA, CA, VME, ME and mE of 97.5%, 94.7%, 1.3%, 0%, and 4.4%, respectively. Conclusions: AXDX had fewer ID failures and more rapid ID/AST results. Direct MALDI/DV2 and AXDX both had high agreement for clinical samples but direct MALDI/DV2 had higher agreement when challenged with MDR GNB.

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MALDI-TOF mass spectrometry for antimicrobial susceptibility testing

Journal of Clinical Microbiology ◽

10.1128/jcm.01814-19 ◽

2021 ◽

Author(s):

Evgeny A. Idelevich ◽

Karsten Becker

Keyword(s):

Mass Spectrometry ◽

Antimicrobial Susceptibility ◽

Susceptibility Testing ◽

Isotope Labeling ◽

Antimicrobial Susceptibility Testing ◽

Maldi Tof Ms ◽

Maldi Target ◽

Maldi Tof ◽

Antibiotic Susceptibility Test ◽

Tof Ms

The advent of matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) in clinical microbiology has dramatically improved the accuracy and speed of diagnostics. However, this progress has mainly been limited to the identification of microorganisms, whereas the practical improvement of antimicrobial susceptibility testing (AST) still lags behind. MALDI-TOF MS-based approaches include the detection of selected resistance mechanisms and the universal phenotypic AST. This minireview focuses on the discussion of those MALDI-TOF MS methods that allow universal growth-based phenotypic AST. The method of minimal profile change concentrations (MPCC) is based on detecting proteome modification in presence of an antimicrobial. Using stable-isotope labeling, characteristic mass shifts in the presence of an antimicrobial indicate the incorporation of the isotopic labels, and, thus, the viability and resistance of the microorganism. For MALDI Biotyper antibiotic susceptibility test rapid assay (MBT-ASTRA), microorganisms are incubated with or without an antimicrobial, followed by cell lysis, protein extraction, and transfer of the cell lysate onto a MALDI target plate. Using the internal standard, peak intensities are correlated to the amount of microbial proteins, and the relative microbial growth is calculated. Most recent development in the field is the direct-on-target microdroplet growth assay (DOT-MGA). Here, incubation of microorganisms with antimicrobials takes place directly on spots of a MALDI target in form of microdroplets. After incubation, nutrient medium is removed by dabbing with absorptive material. Resistant microorganisms grow despite the presence of antimicrobial, and their amplified biomass is detected by MALDI-TOF MS. Finally, an outlook is provided for further assay improvements.

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