Susceptibility Reporting When the Automated Panel Is Silent
A 71-year-old in the ICU has a bloodstream infection. The automated susceptibility panel reports the isolate as resistant to the usual first-line agents but gives no result for a reserve agent because that drug is not on the panel. The patient is deteriorating on the current regimen. The laboratory has the reserve agent available for disk diffusion and gradient-diffusion testing but does not run it routinely. Pharmacy asks whether the laboratory can provide a result within 24 hours. The laboratory director notes that the CLSI breakpoints for this organism-drug combination were revised two years ago and the current software version predates the revision.
1. What should the laboratory do to provide a defensible result quickly, and what quality checks are essential?
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The laboratory can perform a supplemental susceptibility test (gradient diffusion or disk diffusion) for the reserve agent, but only with explicit quality controls. Essential steps: (1) confirm the current CLSI breakpoint for this organism-drug combination and apply the revised breakpoint, not the software's older value-this may require manual interpretation; (2) run the appropriate quality control organism(s) in the same run; (3) use a standardized inoculum and follow the method's incubation conditions exactly; (4) have a qualified technologist or the director review the result before release.
The result should be reported with a comment that it was performed by a supplemental method and interpreted using current CLSI criteria. The laboratory should also flag the software/breakpoint mismatch to the LIS or vendor so future results are not silently interpreted with obsolete breakpoints. A rapid turnaround is appropriate here because the result changes therapy, but speed cannot come at the cost of the controls.
2. How should the laboratory communicate uncertainty, and what is the stewardship implication?
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The report should state the method, the breakpoint source, and any interpretive caveat (e.g., 'supplemental testing; interpret with current CLSI breakpoints'). If the result is borderline or the method has known limitations for this organism, the laboratory should say so rather than presenting a bare category. Direct communication to the prescriber and pharmacy is warranted because the result is time-critical.
The stewardship implication: a result that unlocks a reserve agent should be paired with a recommendation-use the narrowest effective agent, confirm the source (line, endocarditis), and plan for repeat cultures to document clearance. The laboratory's job is to deliver an accurate, interpretable result fast; the stewardship team's job is to translate it into the right regimen. The two should be coordinated, not sequential.