title: Challenges for accurate susceptibility testing, detection and interpretation of β-lactam resistance phenotypes in Pseudomonas aeruginosa: results from a Spanish multicentre study.
abstract: OBJECTIVE
  To evaluate the proficiency of Spanish laboratories regarding accurate susceptibility testing, detection and interpretation of Pseudomonas aeruginosa β-lactam resistance phenotypes.
  
  
  METHODS
  Thirteen characterized strains were sent to 54 participating centres: clinical strains producing horizontally acquired β-lactamases [extended-spectrum β-lactamases (ESBLs; PER-1 and OXA-161) and class A (GES-5) and B (VIM-2) carbapenemases] and mutants with combinations of chromosomal mechanisms (AmpC, OprD and/or efflux). The centres were requested to evaluate six antipseudomonal β-lactams, provide raw/interpreted clinical categories and detect/infer the resistance mechanisms. Consensus results from reference centres were used to assign minor, major or very major errors (mEs, MEs or VMEs).
  
  
  RESULTS
  Vitek2, MicroScan WalkAway and Wider were the most used devices (25%-30% each). CLSI/EUCAST breakpoints were used in 86%/14% of the determinations. Discrepancies exclusively due to the differential application of breakpoints were highest for aztreonam, followed by piperacillin/tazobactam. The lowest percentage of VMEs was for Vitek2, followed by Wider. The highest percentages of VMEs (6%) were for the AmpC-hyperproducing OprD(-) strain and for the GES-5 producer, while among antibiotics the highest percentage of VMEs (22%) involved piperacillin/tazobactam. Appropriate inference of resistance mechanisms was high for the VIM-2-producing strain (83%), but low (<40%) for strains producing ESBLs or non-metallo-β-lactamase carbapenemases.
  
  
  CONCLUSIONS
  The use of different breakpoints and devices, the complexity of mutation-driven resistance mechanisms and the lack of unequivocal tests to detect ESBLs or carbapenemases in P. aeruginosa leads to extraordinary variability and low accuracy in susceptibility testing, which may have consequences for the treatment and control of nosocomial infections.
authors.affiliation: Servicio de Microbiología,  Hospital Son Espases, Palma de Mallorca,  Spain.
authors.forename: Carlos
authors.initials: C
authors.lastname: Juan
authors.forename: M Carmen
authors.initials: MC
authors.lastname: Conejo
authors.forename: Nuria
authors.initials: N
authors.lastname: Tormo
authors.forename: Concha
authors.initials: C
authors.lastname: Gimeno
authors.forename: Álvaro
authors.initials: Á
authors.lastname: Pascual
authors.forename: Antonio
authors.initials: A
authors.lastname: Oliver
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D006801:Humans; D059021:Laboratory Proficiency Testing / Q000458:organization & administration*; D008826:Microbial Sensitivity Tests / Q000379:methods; D010641:Phenotype; D011550:Pseudomonas aeruginosa / Q000187:drug effects*; D013030:Spain; D018440:beta-Lactam Resistance; D001618:beta-Lactamases / Q000235:genetics / Q000378:metabolism; D047090:beta-Lactams / Q000494:pharmacology*
chemical_list: D000900:Anti-Bacterial Agents; D047090:beta-Lactams; D001618:beta-Lactamases