Bacteremia is a worldwide cause of hospitalization and any kind of delay in appropriate antibiotherapy could be harmful or even fatal for the patient [
16]. The wait for both identification and AST results from positive blood cultures can lead to a broad spectrum or ineffective antibiotherapy and exposes the patient to the emergence of multi-resistant bacteria [
17], morbidity and mortality [
18,
19]. Speeded-up positive blood culture testing is therefore an important challenge for the hospital microbiology laboratory. Many authors focused on rapid identification by MALDI-TOF MS directly on positive blood cultures [
7,
20]. However, publications about direct AST methods are less prevalent and mainly address testing on AST automated systems. Beuving et al. evaluated direct inoculation of the Phoenix from positive blood cultures and showed a CA of 95.4% [
21]. Similarly Pan et al. performed Vitek AST on positive blood cultures resulting into a 96.9 and 92.8% CA for respectively Gram-negative and Gram-positive bacteria [
22]. At present many microbiology laboratories have introduced this direct AST approach in routine management of positive blood cultures subsequently reducing TAT to antimicrobial results with 24 h. Nonetheless results remain unavailable on the day of blood culture positivity detection. To this end, Alifax has developed an innovative AST approach based on light scattering measurements detecting the absence/presence of bacteria in a drug suspension within a few hours. In this study the Alfred 60
AST system and 8 selected antibiotics were challenged with 275 positive blood cultures and results were compared to those obtained with direct Phoenix testing in terms of microbiological performances and TAT. Alfred AST results for Gram-negative bacteria during period I showed moderate performances with a CA of 89.5% which is below the Cumitech acceptable performance rates [
15]. Other authors reported similar to slightly higher CA results ranging between 87.7 and 97.7% [
12,
23,
24]. In our study discrepancies were mainly associated with cefuroxime and piperacillin-tazobactam EB testing. Giordano et al. reported similar results for piperacillin-tazobactam EB with a CA for this antibiotic of 77.3% including 7 ME and 3 mE [
24]. Conversely, this team did not observe any errors concerning cefuroxime however only 4 strains were tested. The significant amount of VME and ME led the Alifax Company to review cefuroxime and piperacillin-tazobactam EB reagents and conducted into the delivery of new antibiotic formulations. Subsequently an additional evaluation was performed on Gram-negative bacteria (Period II) with the absence of VME for both antibiotics and an improved global CA of 92.2% considered as adequate according to the Cumitech acceptable performance rates [
15]. Ultimately remaining ME and mE were majorly linked to ceftazidime EB results. Our first hypothesis that errors might have been linked to the variable expression of an ESBL enzyme was countered as only 3/15 mE and 0/11 ME were associated with ESBL strains. We therefore suppose the Alfred 60
AST ceftazidime EB antibiotic was too weakly concentrated leading to false resistance results. Nevertheless cefotaxime is globally more sensitive for the detection of ESBL producers and should be included in the Gram-negative Alfred 60
AST panel when applied in routine to avoid clinical failure with third generation cephalosporins particularly for CTX-M producing Enterobacteriaceae that are cefotaxime resistant but ceftazidime susceptible. Despite only 2 ME concerning ceftazidime PA and a complete concordance concerning ciprofloxacin, AST results for
P. aeruginosa strains are of little value as only 11 strains could be evaluated. Barnini et al. who similarly studied a population of 12
P. aeruginosa strains on Alfred 60
AST showed a total CA of 89.3% for amikacin, colistin, gentamicin, levofloxacin yet ceftazidime PA was not tested [
23]. CA for AST on Gram-positive bacteria with Alfred 60
AST was 88.1% essentially due to a ME rate as high as 42% for cefoxitin tested on coagulase-negative Staphylococci and hereby not meeting the Cumitech acceptable performance rates [
15]. Other authors obtained a Gram-positive CA between 85.1 and 93.7% [
23,
24]. Similarly Barnini et al. reported cefoxitin ME rates for Staphylococci of 14.3 and 17.2% with 2 Alfred 60
AST test protocols [
23]. Inadequately suppressing the antibiotic option of a small spectrum beta-lactam due to erroneous cefoxitin resistance detection could lead to the excess use of broad-spectrum antibiotics including vancomycin. Therefore we believe a review of the composition of the cefoxitin antibiotic for coagulase-negative Staphylococci should be considered. Finally considering the
S. aureus population, cefoxitin showed an optimal CA as observed by others [
24]. This is of importance, mainly because the rapid detection of a blood infection by a methicillin-resistant
S. aureus requires the rapid instauration of a broad-spectrum antibiotherapy [
25].
Our study included some drawbacks. At first our positive blood culture collection was low in multi-resistant strains and additional testing needs to be performed on methicillin-resistant S. aureus, ESBL and carbapenemase-producing Enterobacteriaceae as well as multi-resistant P. aeruginosa for a more accurate evaluation of VME rates. Alongside our evaluation of Alfred 60AST was restricted to a limited panel of antibiotics chosen in accordance with our local resistance epidemiology. In a setting with high prevalence rates of multi-resistant bacteria, meropenem and vancomycin must be part of the customized antibiotic panels. Finally caution is required when global CA, VME, ME and mE are compared between publications as every team evaluates distinct antibiotic panels with different strains including varying resistance profiles.
The phase following this microbiological performances evaluation would be the introduction of Alfred 60
AST testing in the routine management of positive blood cultures with the assessment of the impact on patient’s outcome. A retrospective cohort study of Menon et al. concluded to a speeded-up antibiotic change in 28% of bacteremia cases through the use of susceptibility testing with disk diffusion directly from positive blood cultures versus testing on subcultured colonies [
26]. In an interventional study, Verroken et al. similarly calculated a time gain of 18.2 h towards optimal antimicrobial treatment with the introduction of a speeded-up positive blood culture workflow including direct MALDI-TOF MS identification, rapid resistance detection testing and direct automated AST [
27]. We believe the routine integration of Alfred 60
AST testing as suggested in Fig.
1 would allow analogous observations with an even shorter TAT towards antibiotic tailoring and a potential impact on patient’s mortality and length of stay. However it is important to recall that in our study Alfred 60
AST testing was limited to positive blood cultures detected positive until 10 AM. Extending the inclusion time frame would concurrently set back the time to available results towards evening/night hours requiring an around-the-clock running laboratory and the full-time accessibility of the clinician in charge of the concerned patient to perform instant antibiotic tailoring.