Breakpoints for predicting Pseudomonas aeruginosa susceptibility to inhaled tobramycin in cystic fibrosis patients: use of high-range Etest strips

Inhaled administration of tobramycin assures high concentrations in cystic fibrotic lungs, improving the therapeutic ratio over that of parenteral tobramycin levels, particularly against Pseudomonas aeruginosa. Conventional Clinical and Laboratory Standards Institute (CLSI; formerly National Committee for Clinical Laboratory Standards) breakpoints only consider parenteral levels and do not take into account these high antimicrobial concentrations. The Spanish Antibiogram Committee (The MENSURA Group) has tentatively defined specific breakpoint values for inhaled tobramycin when testing P. aeruginosa isolates from cystic fibrosis (CF) patients (susceptible, < or =64 microg/ml; resistant, > or =128 microg/ml). The antimicrobial susceptibilities of 206 prospectively collected CF P. aeruginosa isolates were determined by the reference agar dilution method. For tobramycin, the performance of high range tobramycin Etest strips (AB Biodisk, Solna, Sweden) and conventional tobramycin disks were assessed with the same collection. Applying MENSURA proposed breakpoints, 95.1% of the strains were categorized as susceptible to tobramycin, either using agar dilution or Etest high-range strips (99% categorical agreement between both methods). With CLSI breakpoints, susceptibility rates decreased to 79.1 and 81.1% for agar dilution and Etest strips, respectively (83.5% categorical agreement). Minor, major, and very major errors for Etest strips (CLSI criteria) were 13.6, 1.2, and 14.8%, respectively. Upon applying the new proposed criteria for inhaled tobramycin, only one major and one very major error were observed with Etest strips. Whenever inhaled tobramycin is considered for therapy, we suggest that P. aeruginosa strains from CF patients categorized as intermediate or resistant to tobramycin according to the CLSI criteria should be retested with high-range Etest strips and recategorized using MENSURA interpretive criteria. CLSI breakpoints should still be followed when intravenous tobramycin is used in CF patients, particularly during the course of exacerbations.