Susceptibility Testing
Diffusion Techniques
Quantitative methods that require measurement of zone diameters provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure
2,4 which has been recommended for use with disks to test susceptibility of organisms to penicillin uses the 10 Unit (U) penicillin disk. Interpretation involves the correlation of the diameters obtained in the disk test with the minimum inhibitory concentration (MIC) for penicillin.
Reports from the laboratory providing results of the standard single-disk susceptibility test with a 10 U penicillin disk should be interpreted according to the criteria provided in Table 1.
Dilution Techniques
Quantitative methods that are used to determine minimum inhibitory concentrations (MICs) provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds. One such standardized procedure
3,4 uses a standardized dilution method (broth or agar) or equivalent with penicillin powder. The MIC values obtained should be interpreted according to the criteria provided in Table 1.
Table 1: SUSCEPTIBILITY TEST INTERPRETIVE CRITERIA Pathogen
| Susceptibility Test Result Interpretive Criteria
|
Disk diffusion (Zone diameter in mm)
| Minimal Inhibitory Concentration (MIC in mcg/mL)
|
| S
| I
| R
| S
| I
| R
|
Staphylococcus spp
.
| ≥29
| -
| ≤28
| ≤0.12
| -
| ≥0.25
|
Streptococcus spp.
(beta-hemolytic group)
| ≥24
| -
| -
| ≤0.12
| -
| -
|
Streptococcus pneumoniae (non-meningitis isolates)
|
|
|
| ≤0.06
| 0.12 to 1
| ≥2
|
A report of “susceptible” indicates that the pathogen is likely to be inhibited by usually achievable concentrations of the antimicrobial compound in the blood. A report of “Intermediate” (I) indicates that the result should be considered equivocal, if the microorganism is not fully susceptible to alternative, clinically feasible drugs, the test should be repeated. This category implies possible clinical applicability in body sites where the drug is physiologically concentrated or in situations where high dosage of the drug can be used. This category also provides a buffer zone that prevents small uncontrolled technical factors from causing major discrepancies in interpretation. A report of “resistant” indicates that the pathogen is not likely to be inhibited if the antimicrobial compound in the blood reaches the concentrations usually achievable; other therapy should be selected
.
Quality Control
Standardized susceptibility test procedures require the use of laboratory control microorganisms
2,3,4. The 10 U penicillin disk and the standard penicillin powder should provide respectively the following zone diameters and MIC values in these laboratory test quality control strains:
Table 2: ACCEPTABLE QUALITY CONTROL RANGES Microorganism
| Acceptable Quality Control Ranges
|
Disk diffusion (Zone diameter ranges in mm)
| Minimal Inhibitory Concentration Range (MIC in mcg/mL)
|
Staphylococcus aureus ATCC
® 25923
| 26 to 37
| |
Staphylococcus aureus ATCC
® 29213
| | 0.25 to 2
|
Streptococcus pneumoniae ATCC
® 49619
| 24 to 30
| 0.25 to 1
|