Reports from the laboratory providing results of the standard single-disk susceptibility test with a 30 µg amoxicillin/ clavulanate acid (20 µg amoxicillin plus 10 µg clavulanate potassium) disk should be interpreted according to the following criteria: RECOMMENDED RANGES FOR AMOXICILLIN/ CLAVULANIC ACID SUSCEPTIBILITY TESTING For Staphylococcus ** species and H. influenzae a : Zone Diameter (mm) Interpretation . 20 Susceptible (S) . 19 Resistant (R) For other organisms except S. pneumoniae b and N. gonorrhoeae c : Zone Diameter (mm) Interpretation . 18 Susceptible (S) 14 to 17 Intermediate (I) . 13 Resistant (R) ** Staphylococci which are resistant to methicillin/ oxacillin must be considered as resistant to amoxicillin/ clavulanic acid. a A broth microdilution method should be used for testing H. influenzae. Beta-lactamase negative, ampicillin-resistant strains must be considered resistant to amoxicillin/ clavulanic acid. b Susceptibility of S. pneumoniae should be determined using a 1 µg oxacillin disk. Isolates with oxacillin zone sizes of . 20 mm are susceptible to amoxicillin/ clavulanic acid. An amoxicillin/ clavulanic acid MIC should be determined on isolates of S. pneumoniae with oxacillin zone sizes of . 19 mm. c A broth microdilution method should be used for testing N. gonorrhoeae and interpreted according to penicillin breakpoints. Interpretation should be as stated above for results using dilution techniques. Interpretation involves correlation of the diameter obtained in the disk test with the MIC for amoxicillin/ clavulanic acid. As with standardized dilution techniques, diffusion methods require the use of laboratory control microorganisms that are used to control the technical aspects of the laboratory procedures. For the diffusion technique, the 30 µg amoxicillin/ clavulanate potassium (20 µg amoxicillin plus 10 µg clavulanate potassium) disk should provide the following zone diameters in these laboratory quality control strains: Microorganism Zone Diameter (mm) Escherichia coli ATCC 25922 19 to 25 Escherichia coli ATCC 35218 18 to 22 Staphylococcus aureus ATCC 25923 28 to 36 | ||||
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