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Objective:
Nationwide trends show rising computed tomography (CT) utilization. Patients with soft tissue infection, including cellulitis, are receiving an increasing number of CT scans, ostensibly in attempt to exclude deep infection. Extremities are a common site of cellulitis, but the value of utilization of CT in this anatomic cohort remains unclear. The purpose of this study is to describe the utilization of CT among patients with extremity cellulitis and evaluate the yield of CT for suspected deep infection in this cohort.

Materials and Methods:
A retrospective observational study was performed at a single level 1 trauma medical center. Eligibility criteria included a medical center encounter involving an adult patient with a clinical diagnosis of cellulitis of an extremity who underwent a CT scan for evaluation of the extremity between 2020 and 2021. Patients with any of the following characteristics of the affected extremity were excluded: concurrent trauma, recent surgery, indwelling implants exclusive of arthroplasty, known deep infection, or malignancy. Logistic regression was used to analyze whether demographic and clinical factors influenced positive findings of deep infection on CT.

Results:
All 144 encounters among 134 patients met inclusion criteria. Mean age at CT was 57±18.8 years of age, mean body mass index (BMI) was 37.9±14 kg/m^2, and 54% of patients were men. In 54% (77/144) of cases, radiographs were not performed prior to CT. Of 66 cases (46%) in which radiography was performed prior to CT, gas was found in 5, swelling in 41, and 20 had neither gas nor swelling. CT was positive for findings of deep infection in 13.2% of all cases (19/144). The odds of findings of deep infection on CT were 16.9 times greater for those with history of injection drug use (95% CI 2.4-121.0, p=0.005), and 16.9 times greater for those with gas on radiographs (95% CI 2.6-110.7, p=0.003) in an adjusted multivariable repeated measures logistic regression model.

Conclusion:
CT utilization is an ongoing area of interest for clinicians to reduce unnecessary testing. In this study, we examined CT utilization for patients with a clinical diagnosis of cellulitis of the extremity, a scenario in which ACR Appropriateness Criteria suggests radiography as an initial exam and CT as usually appropriate only in cases of gas or swelling found on radiography. In this cohort of 144 CTs, approximately one-third adhered to Appropriateness Criteria. In over half of the cases, CT was used as the initial imaging study. Of patients who did undergo radiography prior to CT, one-third subsequently received a CT despite absence of appropriate radiography findings. Our finding that injection drug use was a significant predictor for positive CT is consistent with previous research identifying this as a risk factor for deep infection. The overall yield of positive deep infection findings on CT was 13.2%. As extremities are readily amenable to physical examination, low yield of CT in this cohort may represent overuse of cross-sectional imaging when clinical diagnosis is sufficient for determination and management of cellulitis.