A susceptibility report answers the questions the laboratory was asked. It is silent on the ones it was not, and that silence is routinely read as evidence of resistance.
Many laboratories suppress results for agents they consider inappropriate for the specimen or the organism, or simply do not include them on the standard panel. An organism reported susceptible only to a carbapenem may be perfectly susceptible to a narrower agent that was never tested. Escalation then happens by default rather than by evidence.
So when a report pushes you towards a broad agent, ring the laboratory before you prescribe it. Ask what was on the panel, what was suppressed and why, and whether the narrower agent can be tested on the stored isolate. It usually can, and it usually takes a day — which is often less time than the course you were about to start.
- Read the panel, not just the results — note which agents are absent, not only which are resistant.
- Ask whether any results were suppressed by laboratory policy before assuming resistance.
- Request testing of a narrower agent on the stored isolate when de-escalation is plausible.
- Record the de-escalation decision and its rationale in the notes so the next clinician does not re-escalate.
- Know your own institution's reporting policy — the suppression rules differ between laboratories, and most prescribers have never seen them.
Why it matters
An untested antibiotic looks identical to a resistant one on the report, and prescribers treat them the same.
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