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Does “bi-weekly” mean twice per week or once every two weeks? The general recommendation by CLSI is to perform routine QC for Disk Diffusion method once per week, after you have documented satisfactory daily performance. To document satisfactory daily results, you must test your QC strains for 20 consecutive days, with no more than one zone diameter out of acceptable range. If you have more than one diameter out of range, continue the daily testing for 10 more consecutive days (30 days total). If no more than 3 out of the 30 zone diameters are out of range, then you can switch to weekly QC testing. “Consecutive days” refer to days on which you test patients; if you do not set up patient isolates every day, your consecutive days might not be every day of the week.
If your testing volume is low (you test patients less than once per week), then you should consider setting up Disk Diffusion QC each day that you test patients. You must also run QC (regardless of testing volume) whenever you change any component (new lot number of agar, new lot number of disks). If you add a new drug to your panel, you must test that drug against the applicable QC strain(s) for 20 or 30 days as described above before you can test it on a weekly schedule. Performing QC once every two weeks would be acceptable only if you are setting up patient isolates once every two weeks on the same day that you are running QC. Please see CLSI document M2-A9 (published in January 2006) for more details.
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