Augmentin - Clinical Pharmacology

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SUSCEPTIBILITY TESTING

Dilution Techniques:

Quantitative methods are used to determine antimicrobial minimal 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 a dilution method 1 (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:

RECOMMENDED RANGES FOR AMOXICILLIN/ CLAVULANIC ACID SUSCEPTIBILITY TESTING

For gram-negative enteric aerobes: MIC (µg/ mL) Interpretation

. 8/ 4 Susceptible (S) 16/ 8 Intermediate (I)

. 32/ 16 Resistant (R) For Staphylococcus ** and Haemophilus species:

MIC (µg/ 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 Streptococcus pneumoniae from non-meningitis sources: Isolates should be tested using amoxicillin/ clavulanic acid and the following criteria should be used:

MIC (µg/ 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 which prevents small uncontrolled technical factors from causing major discrepancies in interpretation.