Post-Operative Fever Cluster on an Orthopedic Unit
Over 11 days, four patients who underwent total knee arthroplasty in the same operating suite develop wound drainage and low-grade fever (38.1-38.6 C) between postoperative days 5 and 9. Three grow Staphylococcus aureus; two are methicillin-resistant. All four had surgery in OR-3. Two of the four had the same surgeon, but the other two did not. The unit's baseline surgical site infection rate for knee arthroplasty is 0.8%. The infection preventionist is asked to determine whether this represents an outbreak and what the next investigative step should be. Water cultures from the OR scrub sink were negative last quarter. The OR-3 ventilation was last certified 9 months ago and passed.
1. Is this an outbreak? Justify your determination using the available data, and identify what additional information would change your answer.
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An outbreak is the occurrence of more cases of disease than expected in a defined group or area over a given period. Here, 4 SSIs in 11 days against a baseline of 0.8% is a substantial excess, and the cases share a plausible common exposure (OR-3). Even before formal statistics, this meets the working definition and warrants an investigation.
However, the IP must confirm the denominator: how many knee arthroplasties were performed in OR-3 during those 11 days? Four cases out of, say, 6 procedures is a very different signal than 4 out of 120. The IP should also verify that the baseline 0.8% is current and comparable (same case mix, same NHSN definition), and confirm that all four isolates are truly distinct patients with independently reviewed case definitions rather than surveillance artifacts.
What would change the answer: if the denominator shows the rate is within expected range, or if the cases fail the NHSN SSI definition on review, the cluster may not be an outbreak. Conversely, if the denominator is small and the rate is far above baseline, the signal strengthens.
2. What is the single most informative next investigative step, and why?
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The most informative next step is to characterize the exposures shared by the cases versus non-cases - specifically, build a line list of all knee arthroplasties in OR-3 during the period, with case status and variables such as surgeon, scrub tech, circulating nurse, anesthesia provider, implant lot, procedure date, and time of day.
The shared-surgeon finding is a red herring until tested against the denominator: two of four cases sharing a surgeon is unremarkable if that surgeon performs most of the cases. The IP needs to know whether the surgeon's proportion of cases among cases exceeds their proportion among non-cases.
This step is more informative than environmental sampling because it tests hypotheses about transmission route and narrows the field before expensive or low-yield testing. Molecular typing (e.g., PFGE or WGS) of the isolates would then confirm whether the cases are clonal, which distinguishes a common-source outbreak from coincidental sporadic infections.
3. The IP proposes immediate closure of OR-3 pending investigation. Evaluate this proposal.
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Closure is a high-consequence action that should be proportionate to evidence. At this point the IP has a plausible but unconfirmed association with OR-3. Before closing, the IP should consider less disruptive steps: intensified surveillance, review of OR-3 practices (sterile technique, traffic, flash sterilization use, instrument reprocessing), and expedited molecular typing.
Closure may be justified if: (1) the denominator confirms an excess rate concentrated in OR-3; (2) a common-source exposure within OR-3 is identified (e.g., a contaminated instrument set, a colonized staff member, a ventilation defect); or (3) the cluster is expanding rapidly. The IP should also weigh the risk of harm from delaying closure against the harm of disrupting care and the possibility that the source is elsewhere (e.g., the surgeon's technique, patient decolonization practices, or the implant).
The decision should be made with the OR leadership and documented with the rationale, and a plan for reopening (corrective actions, verification) should be defined in advance.