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Related Experiment Videos

Species-specific interpretive breakpoints for ciprofloxacin disk diffusion susceptibility testing.

S Ringertz1, A Björklind, G Kronvall

  • 1Department of Clinical Microbiology, Karolinska Hospital, Karolinska Institute, Stockholm, Sweden.

Scandinavian Journal of Infectious Diseases. Supplementum
|January 1, 1989
PubMed
Summary

Routine susceptibility testing for ciprofloxacin showed low accuracy. New, species-specific breakpoints were determined to improve the reliability of disk diffusion tests for various bacteria, including Pseudomonas species.

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Area of Science:

  • Clinical Microbiology
  • Antimicrobial Susceptibility Testing
  • Pharmacology

Background:

  • Initial interpretive standards for ciprofloxacin disk diffusion testing in Sweden exhibited low accuracy.
  • A quality control study identified significant discrepancies in classifying bacterial susceptibility to ciprofloxacin.

Purpose of the Study:

  • To re-evaluate and establish accurate, species-specific breakpoints for ciprofloxacin disk diffusion susceptibility testing.
  • To enhance the reliability of routine antimicrobial susceptibility testing.

Main Methods:

  • Disk diffusion susceptibility testing using 10 microgram ciprofloxacin disks.
  • Determination of new breakpoints using the SRA (Species-specific Resistance Analysis) method.
  • Evaluation of accuracy with various bacterial strains, including Staphylococcus aureus, Streptococcus agalactiae, Enterococci, Pseudomonas aeruginosa, and Pseudomonas maltophilia.

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Main Results:

  • Initial breakpoints led to frequent misclassification of susceptible strains as intermediate for Staphylococcus aureus and Streptococcus agalactiae.
  • The test failed to detect true resistance in Pseudomonas maltophilia with the original breakpoints.
  • New proposed breakpoints (S ≥ 20 mm, R < 13 mm for most bacteria; S ≥ 27 mm, R < 18 mm for P. aeruginosa; S ≥ 30 mm, R < 23 mm for P. maltophilia) demonstrated acceptable accuracy.
  • Streptococcus faecalis strains predominantly fell into the intermediate category.

Conclusions:

  • The original interpretive standards for ciprofloxacin disk diffusion were inadequate.
  • Species-specific breakpoints are necessary for accurate susceptibility testing of ciprofloxacin, particularly for Pseudomonas species.
  • The proposed new breakpoints improve the accuracy of routine ciprofloxacin susceptibility testing.