Antimicrobial susceptibility testing using disc diffusion methods, C.


 

 

 

 

 

 

 

Calibration of the disc diffusion test and determination of optimal disc content for routine antimicrobial susceptibility testing.

From the SRA experience it was clear that interpretive zone diameter breakpoints could be determined corresponding to MIC limits set by reference authorities for individual bacterial species using SRA, single strain regression analysis. This means, in fact, that the disc diffusion test CAN BE CALIBRATED, not only for drug-bug combinations but also for individual laboratories. Calibration procedures have been routine tasks in clinical chemistry laboratories for decades.

Examples of calibration procedures using SRA will be given.

When a clinical microbiology laboratory in Tartu, Estonia, wanted to set up fusidic acid disc diffusion tests for isolated Staphylococcus aureus strains, they used the NCCLS standard which lacked zone breakpoints for this antibiotic. On the other hand, SRGA issued both MIC limits and zone interpretive breakpoints, the latter though for a disc content of 50 µg. In Tartu they wanted to use a lower disc concent, 10 µg. The solution was to calculate the new zone breakpoints using SRA.

Another example provided zone breakpoints for some anaerobic species and trovafloxacin susceptibility. Although this fluoroquinolone is not available on the market, the procedure can be applied to other new antimicrobials at some stage in the clinical testing.

There are often arguments regarding disc contents for routine clinical laboratory disc testing. The power of SRA calculations can actually provide a new definition of the optimal disc content for diffusion tests:

"The lowest disc content of an antibiotic which will distinguish resistant strains of any bacterial species from strains of the intermediate or susceptible category."

This is possible to determine using SRA as was shown for fusidic acid and S.aureus , for trovafloxacin and aerobic pathogens , and for trovafloxacin and anaerobes.

Calibration was a valuable feature of SRA, the equation obtained from original formulae describing the disc diffusion test. A further extension of the SRA equation leads to the so called M-test, where you can determine the MIC value of an isolate using several disc contents,
(see next page).

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Page updated by G. Kronvall, 2015-10-09