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To reduce the development of drug-resistant bacteria and maintain the effectiveness of amoxicillin and clavulanate potassium for oral suspension and chewable tablets and other antibacterial drugs, amoxicillin and clavulanate potassium for oral suspension and chewable tablets should be used only to treat or prevent infections that are proven or strongly suspected to be caused by bacteria.
Susceptibility TestingDilution TechniquesQuantitative methods are used to determine antimicrobial 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 a dilution method1 (broth or agar) or equivalent with standardized inoculum concentrations and standardized concentrations of amoxicillin/clavulanate potassium powder.
The recommended dilution pattern utilizes a constant amoxicillin/clavulanate potassium ratio of 2 to 1 in all tubes with varying amounts of amoxicillin. MICs are expressed in terms of the amoxicillin concentration in the presence of clavulanic acid at a constant 2 parts amoxicillin to 1 part clavulanic acid. The MIC values should be interpreted according to the following criteria:
| For gream-negative enteric aerobes: | |
| MIC (mcg/mL) | Interpretation |
| ≤8/4 | Susceptible (S) |
| 16/8 | Intermediate (I) |
| ≥32/16 | Resistant (R) |
| For Staphylococcus * and Haemophilus species: | |
| MIC (mcg/mL) | Interpretation |
| ≤4/2 | Susceptible (S) |
| ≥8/4 | Resistant (R) |
Staphylococci which are susceptible to amoxicillin/clavulanic acid but resistant to methicillin/oxacillin must be considered as resistant.
For S. pneumoniae from non-meningitis sources: Isolates should be tested using amoxicillin/clavulanic acid and the following criteria should be used:
| MIC (mcg/mL) | Interpretation |
| ≤2/1 | Susceptible (S) |
| 4/2 | Intermediate (I) |
| ≥8/4 | Resistant (R) |
Note: These interpretive criteria are based on the recommended doses for respiratory tract infections.
A report of "Susceptible" indicates that the pathogen is likely to be inhibited if the antimicrobial compound in the blood reaches the concentration usually achievable. A report of "Intermediate" indicates that the result should be considered equivocal, and, 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 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.
Standardized susceptibility test procedures require the use of laboratory control microorganisms to control the technical aspects of the laboratory procedures. Standard amoxicillin/clavulanate potassium powder should provide the following MIC values:
| Microorganism | MIC Range (mcg/mL) * |
| Escherichia coli ATCC 25922 | 2 to 8 |
| Escherichia coli ATCC 35218 | 4 to 16 |
| Enterococcus faecalis ATCC 29212 | 0.25 to 1 |
| Haemophilus influenzae ATCC 49247 | 2 to 16 |
| Staphylococcus aureus ATCC 29213 | 0.12 to 0.5 |
| Streptococcus pneumoniae ATCC 49619 | 0.03 to 0.12 |
Expressed as concentration of amoxicillin in the presence of clavulanic acid at a constant 2 parts amoxicillin to 1 part clavulanic acid. 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 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 30 mcg of amoxicillin/clavulanate potassium (20 mcg amoxicillin plus 10 mcg clavulanate potassium) to test the susceptibility of microorganisms to amoxicillin/clavulanic acid.
Reports from the laboratory providing results of the standard singledisk susceptibility test with a 30 mcg amoxicillin/clavulanate potassium (20 mcg amoxicillin plus 10 mcg clavulanate potassium) disk should be interpreted according to the following criteria:
| For Staphylococcus * species and H. influenzae†: | |
| Zone Diameter (mm) | Interpretation |
| ≥20 | Susceptible (S) |
| ≤19 | Resistant (R) |
| For other organisms except S. pneumoniae‡ and N. gonorrhoeae§: | |
| Zone Diameter (mm) | Interpretation |
| ≥18 | Susceptible (S) |
| 14 to 17 | Intermediate (I) |
| ≤13 | Resistant (R) |
† 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.
‡ Susceptibility of S. pneumoniae should be determined using a 1 mcg 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.
§ 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 mcg amoxicillin/clavulanate potassium (20 mcg amoxicillin plus 10 mcg 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 mm |
| Escherichia coli ATCC 35218 | 18 to 22 mm |
| Staphylococcus aureus ATCC 25923 | 28 to 36 mm |