Evaluation of the performance of manual antimicrobial susceptibility testing methods and disk breakpoints for stenotrophomonas maltophilia

Ayesha Khan, Cedric H. Pettaway, Jennifer Dien Bard, Cesar A. Arias, Micah M. Bhatti, Romney M. Humphries

Research output: Contribution to journalArticlepeer-review

2 Scopus citations


Stenotrophomonas maltophilia is an emerging cause of serious infections with high associated mortality in immunocompromised patients. Treatment of S. maltophilia infections is complicated by intrinsic resistance to many antimicrobials, including carbapenems, aminoglycosides, and some cephalosporins. Despite this, .90% of isolates are susceptible to trimethoprim-sulfamethoxazole (SXT), which is the front-line therapy for this organism. Side effects of SXT include bone marrow suppression, which precludes its use for many neutropenic patients. In this population, levofloxacin (LEV), minocycline (MIN), ceftazidime (CAZ), ciprofloxacin (CIP), and tigecycline (TIG) are used as alternative therapies, all of which require testing to inform susceptibilities. The reference standard method for testing S. maltophilia is broth microdilution (BMD), but very few clinical laboratories perform reference BMD. Furthermore, interpretive criteria are not available for CIP or TIG for S. maltophilia, although generic pharmacokinetic/pharmacodynamic (PK/PD) MIC breakpoints are available for these drugs. We assessed performance of disk and gradient diffusion tests relative to BMD for 109 contemporary isolates of S. maltophilia. Categorical agreement values for SXT, LEV, and MIN disk diffusion were 93%, 89%, and 95%, respectively. Categorical agreement values for SXT, LEV, MIN, and CAZ gradient strips were 98%, 85%, 93%, and 71%, respectively, by Etest (bioMerieux) and 98%, 83%, 99%, and 73% by the MIC test strip (MTS; Liofilchem). CIP and TGC, two clinically valuable alternatives to SXT, did not demonstrate promising disk-to-MIC correlates using CLSI document M100-ED30 (Clinical and Laboratory Standards Institute, Performance Standards for Antimicrobial Susceptibility Testing, M100-ED30, 2020) P. aeruginosa or PK/PD breakpoints. Manual commercial tests perform well for S. maltophilia, with the exception of tests for LEV and CAZ, for which high error rates were observed.

Original languageEnglish (US)
Article numbere02631-20
JournalAntimicrobial Agents and Chemotherapy
Issue number5
StatePublished - May 2021


  • Antibiotic resistance
  • Antimicrobial agents
  • Blood culture
  • Bloodstream infections
  • Diagnostics
  • Gram-negative bacteria
  • Immunocompromised hosts
  • Opportunistic infections
  • Stenotrophomonas
  • Susceptibility testing

ASJC Scopus subject areas

  • Pharmacology
  • Pharmacology (medical)
  • Infectious Diseases


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