CLSI definitions for Succeptibility testing - Lab week 7

As everyone remembers, we  have done susceptibility testing this week during lab. 

The test is performed by applying a bacterial inoculum to the surface of a large Mueller-Hinton agar plate. After the plates have been inoculated  for 16–24 h at 35°C prior to determination of results, the susceptibility of each on the antimicrobials was graded according to the diameter of the inhibition zone, and then susceptibility was graded as either susceptible, intermediate and resistant. The diameter of the zone is related to the susceptibility of the isolate and to the diffusion rate of the drug through the agar medium.

According to the CLSI, those categories each has a certan clinical implication:

Susceptible

'The "susceptible" category implies that isolates are inhibited by the usually achievable concentrations of antimicrobial agent when the recommended dosage is used for the site of infection.

 

Resistant

'The "resistant" category implies that isolates are not inhibited by the usually achieveable concentrations of the agent with normal dosage schedules, and/or that demonstrate zone diameters that fall in the range where specific microbial resistance mechanisms (e.g. beta-lactamases) are likely, and clinical efficacy of the agent against the isolate has not been reliably shown in treatment studies.

 

Intermediate

'The "intermediate" category includes isolates with antimicrobial MICs that approach usually attainable blood and tissue levels and for which response rates may be lower than for susceptible isolates. The intermediate category implies clinical efficacy in body sites where the drugs are physiologically concentrated. This category also includes a buffer zone, which should prevent small, uncontrolled, technical factors from causing major discrepancies in interpretations, especially for drugs with narrow pharamacotoxicity margins.