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Hello, we are going to start using the E-Test method for susceptibility for S. maltophilia isolated on patient's cultures. We have elected to set up the Q.C. when we set up the patient test since we do not encounter S. maltophilia very often. My question is do we need to set up both E.coli and Ps. aeruginosa ATCC organisms or will using only one ATCC organism be sufficient for QC? The E-Test is very expensive and would like to see if I can be as cost efficient as possible and have the E-Test last as long as possible. Perhaps run both ATCC organsims for 5 days and if ok, then run E.coli after that with patient.
Thank you, Elisa
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Quality control (QC) for MIC testing (including the agar gradient diffusion method, comercially marketed as the E-test) of Stenotrophomonas maltophilia is given in Table 2B-4 in the CLSI M100-S16 document. Both Escherichia coli ATCC 25922 and Pseudomonas areuginosa ATCC 27853 would have to be tested. E. coli ATCC 35218 would also have to be used if ticarcillin-clavulinic acid was also being tested. The main drawback of QC testing with each isolate tested is that testing may be delayed a day in order to produce fresh subcultures of the QC strains. Sine E-test strips are only sold in packs of 100 strips, you might have issues with outdated strips when testing is done infrequently. You might consider using disk testing for minocylcine, levofloxacin and triment-sulfa and only using the E-strips for ceftazidime, ticarcillin-clavulinic acid and chloramphenicol if you want to test all the antimicrobials for which interbretative criteris (breakpoints) are provided in the M100 document. Another option would be to use dried or frozen microtiter trays or individual microwell strips which contain dilutions of a single antimicrobial. This issue of susceptibility testing of infrequently isolated organisms is one that all clinical labs struggle with and what works best for one lab may not be best for other labs.
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