University of Pennsylvania Medical Center Guidelines for Antibiotic Use

Antimicrobial Susceptibility Testing

What does it mean?

When a microorganism is isolated from a patient, the microbiology lab will often perform susceptibility testing. There is often confusion about what these results mean and how it can be used by the clinician to guide the treatment of the patient.

To find out what these test are and what they mean......

Read and follow the links below

What is the goal of testing?

The goal of antimicrobial susceptibility testing is to predict the in vivo success or failure of antibiotic therapy. Tests are performed in vitro, and measure the growth response of an isolated organism to a particular drug or drugs. The tests are performed under standardized conditions so that the results are reproducible. The test results should be used to guide antibiotic choice. The results of antimicrobial susceptibility testing should be combined with clinical information and experience when selecting the most appropriate antibiotic for your patient.

What is the output from antimicrobial susceptibility testing?

The raw data are either in the form of a zone size or MIC. CLSI has published interpretation criteria for these tests, which has come from extensive testing and clinical correlation.

How are the results reported?

Typically, the raw data are interpreted based on the available CLSI data. The results will be reported out as:

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.' (CLSI definition)

 

 

 

Note that this definition says nothing about the chances of clinical success; in fact predicting clinical outcome based on susceptibility testing and the use of drugs shown to be in the susceptible category is very imprecise. This imprecision is due to the effect of host responses, site of infection, toxin production by bacteria that is independent of antimicrobial susceptibility, the presence of biofilm, drug pharmacodynamics and other factors.

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.' (CLSI definition).

 

Note that this definition says nothing about the chances of clinical success; in fact predicting clinical outcome based on susceptibility testing and the use of drugs shown to be in the resistant category is imprecise. This imprecision is due to the effect of host responses, site of infection, toxin production by bacteria that is independent of antimicrobial susceptibility, the presence or absence of biofilm, drug pharmacodynamics and other factors. However, with the exception of urinary bladder infections and some mycobacterial infections, most clinicians avoid the use of a "resistant" category drug to treat infection.

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 (e.g. quinolones and beta-lactams in urine) or when a higher than normal dosage of a drug can be used (e.g. betalactams). 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.' (CLSI definition)

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The Interpretation of Antimicrobial Susceptibility Testing pages were written by Bruce Sachais 6/17/97, and updated by Paul Edelstein in August 2007.