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

When ciprofloxacin was introduced on the Swedish market the recommended interpretive standards for disk diffusion susceptibility testing using a 10 microgram disk were: Sensitive greater than or equal to 24 mm (MIC less than or equal to 1 mg/l) and Resistant less than 18 mm (MIC greater than 4 mg/l), except for enterococci where S greater than or equal to 18 mm and R less than 14 mm were recommended. A quality control study revealed that the accuracy of the routine susceptibility testing of ciprofloxacin was low. 50% of the strains of Staphylococcus aureus and 90% of Streptococcus agalactiae strains were falsely assigned to the intermediate instead of the sensitive category, and the test missed to detect true resistance in Pseudomonas maltophilia. New species-specific breakpoints were determined by the SRA method. Separate breakpoints were required only for the pseudomonas species. Streptococcus faecalis was found to belong mainly to the intermediate group and it is suggested that these strains are not categorized as sensitive. The breakpoints that gave acceptable accuracy in routine susceptibility testing of ciprofloxacin were S greater than or equal to 20 mm, R less than 13 mm for all relevant bacteria except Pseudomonas aeruginosa for which S greater than or equal to 27 mm, R less than 18 mm is proposed, and Pseudomonas maltophilia where S greater than or equal to 30 mm, R less than 23 mm are adequate breakpoints. The 10 microgram disk gave acceptable inhibition zones for all strains with MICs within the clinically interesting MIC range.