“The point of moving this requirement and revising the note is that the lab should think not just about bacteria,” Dr. Rauch says. “If you’re doing testing on yeast, you will want to use the same process that you use for your much more numerous bacteria.” The report, she notes, does more than help a physician choose a drug empirically. “It also helps the stewardship team decide which drugs the hospital pharmacy should stock.”
Also in the new subsection is MIC.12060 Inconsistent Antimicrobial Susceptibility Testing Results, which requires the laboratory to investigate unusual or inconsistent results using a defined process. Here, too, the scope has been broadened to include yeasts and molds, with the requirement noting that the time of endpoint reading (particularly for the echinocandins) and the effect of trailing growth (particularly for the azoles and flucytosine) can be significant factors that affect susceptibility results. Examples of inconsistent antifungal results are provided, such as Candida albicans resistant to all azoles. An inconsistent antimycobacterial test result would be Mycobacterium bovis/Mycobacterium bovis BCG susceptible to pyrazinamide.
“You might also want to consider talking to the clinician who is anxiously awaiting the test result to let them know you need to figure it out first,” Dr. Rauch says of such results and the required investigation. The inconsistency might be between two results, she notes, or an inconsistency with the pattern generally seen in the laboratory.
MIC.12030 Testing and Reporting Supplemental Antimicrobial Agents is in the new subsection with the abovementioned requirements, but it has been moved and renumbered only, not revised. It requires the laboratory to provide supplemental agent testing for organisms resistant to routinely tested antimicrobial agents when necessary.
“All of these requirements existed before,” Dr. Rauch says of the susceptibility subsection. “But they’ve been revised and moved to where you see them together as a group.”
In the microbiology checklist, the bacteriology susceptibility test QC frequency requirement (MIC.21910) has been revised in the 2025 edition.
Laboratories can now extend external QC frequency up to monthly, with proper risk assessment and QC study performance, if the laboratory director approves the approach through the use of an individualized quality control plan for susceptibility testing.
“There is a lot more expanded guidance,” Dr. Rauch says of the revised requirement. “It explains the pathway to decrease the frequency.”
Satisfactory performance criteria are defined for a 20- or 30-day plan and for a 15-replicate (3 × 5-day) plan. The maximum interval between runs of external QC must be justified by the QC study. For example, a 3 × 5 QC study cannot be used alone to support a monthly QC frequency, and a 20-day study cannot justify longer than a 20-day external QC interval without additional data. Laboratories with existing IQCPs can use historical data in their risk assessments.
When reducing the frequency of QC testing for susceptibility testing, the requirement says, there are many items to consider for an IQCP in addition to satisfactory performance of QC data, among them an assessment of user QC responsibilities, such as inoculum, reading of susceptibility tests, and other QA processes such as training.
The same is noted for antifungal susceptibility testing in MIC.42600 Susceptibility Testing QC Frequency, which is in the mycology susceptibility testing section. “It remains in the mycology section because these rules are unique to fungi,” Dr. Rauch says. Here, too, there is new detail on QC study performance, but the maximum interval for QC performance remains weekly.