nicillins are active in vitro against a variety of other bacteria.
Susceptibility Test Methods
When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice areas to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens. These reports should aid the physician in selecting the most effective antimicrobial.
Dilution Techniques
Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs). These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds. The MICs should be determined using a standardized procedure. Standardized procedures are based on dilution method 1, 2 (broth, agar or microdilution) or equivalent using standardized inoculum and concentrations of oxacillin. The MIC values should be interpreted according to the criteria in Table 1.
Diffusion Techniques
Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure2,3 requires the use of standardized inoculum concentrations. It has been determined that the most accurate method to test the susceptibility of microorganisms to penicillinase-resistant penicillins, including oxacillin, by disk diffusion is achieved using disks impregnated with 30 mcg cefoxitin. Interpretation involves correlation of the diameter obtained in the cefoxitin disk test with the MIC for oxacillin.2,4,5 Reports from the laboratory providing results of the standard single-disk susceptibility test with a 30 microgram cefoxitin disk should be interpreted according to the following criteria in Table 1.
Table 1: Susceptibility Test Interpretive Criteria for Oxacillin
|
|
Minimum Inhibitory Concentrations (mcg/mL) |
Cefoxitin Disk Diffusion Diameters (mm) |
|
Pathogen |
Susceptible (S) |
Resistant (R) |
Susceptible (S) |
Resistant (R) |
|
Staphylococcus aureus, S. lugdunensis |
≤ 2 |
≥ 4 |
≥ 22 |
≤ 21 |
Staphylococci
(coagulase negative) |
≤ 0.25 |
≥ 0.5 |
≥ 25 |
≤ 24 |
A report of “Susceptible” indicates that the pathogen is likely to be inhibited by usually achievable concentrations of the antimicrobial compound in blood. A report of “Resistant” indicates that usually achievable concentrations of the antimicrobial compound in the blood are unlikely to be inhibitory and that other therapy should be selected.
Quality Control
Standardized susceptibility test procedures require the use of laboratory control microorganisms to monitor and ensure the accuracy and precision of the supplies and reagents used in the assay, and the techniques of the individuals performing the test. Standard oxacillin powder should provide MIC values provided below. For the diffusion technique, the 30 mcg cefoxitin
Table 2. In Vitro Susceptibility Test Quality Control Ranges for Oxacillin
|
