Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure² requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 5-μg ofloxacin to test the susceptibility of microorganisms to ofloxacin.

Reports from the laboratory providing results of the standard single-disk susceptibility test with a 5-μg ofloxacin disk should be interpreted according to the following criteria:

For testing Enterobacteriaceae, methicillin-susceptible Staphylococcus aureus, and Pseudomonas aeruginosa:

For testing Haemophilus influenzae:^g

The current absence of data on resistant strains precludes defining any results other than “Susceptible”. Strains yielding zone diameter results suggestive of a “nonsusceptible” category should be submitted to a reference laboratory for further testing.

For testing Neisseria gonorrhoeae:^h

For testing Streptococcus pneumoniae and Streptococcus pyogenes:ⁱ

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 ofloxacin.

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 5-μg ofloxacin disk should provide the following zone diameters in these laboratory quality control strains:

ANIMAL PHARMACOLOGY

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