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Journal of Clinical Microbiology, September 2005, p. 4480-4485, Vol. 43, No. 9
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.9.4480-4485.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.

Breakpoints for Predicting Pseudomonas aeruginosa Susceptibility to Inhaled Tobramycin in Cystic Fibrosis Patients: Use of High-Range Etest Strips

María I. Morosini,1 María García-Castillo,1 Elena Loza,1 María Pérez-Vázquez,2 Fernando Baquero,1 and Rafael Cantón1*

Servicio de Microbiología, Hospital Universitario Ramón y Cajal, Madrid,1 Centro Nacional de Microbiología, Instituto de Salud Carlos III, Majadahonda, Madrid, Spain2

Received 9 March 2005/ Returned for modification 20 May 2005/ Accepted 14 June 2005

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, ≤64 µg/ml; resistant, ≥128 µg/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.

* Corresponding author. Mailing address: Servicio de Microbiología, Hospital Universitario Ramón y Cajal, 28034 Madrid, Spain. Phone: 34-913368330. Fax: 34-913368809. E-mail: rcanton.hrc{at}salud.madrid.org.

Journal of Clinical Microbiology, September 2005, p. 4480-4485, Vol. 43, No. 9
0095-1137/05/$08.00+0     doi:10.1128/JCM.43.9.4480-4485.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.



This article has been cited by other articles:

  • Strateva, T., Petrova, G., Perenovska, P., Mitov, I. (2009). Bulgarian cystic fibrosis Pseudomonas aeruginosa isolates: antimicrobial susceptibility and neuraminidase-encoding gene distribution. J Med Microbiol 58: 690-692 [Full Text]  
  • Govan, J. R. W. (2006). Multidrug-resistant pulmonary infection in cystic fibrosis - what does 'resistant' mean?. J Med Microbiol 55: 1615-1617 [Full Text]  


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