CT utilization in evaluation of skin and soft tissue extremity infections in the ED: Retrospective cohort study

a b s t r a c t

Objective: skin and soft tissue infections (SSTI) are commonly diagnosed in the emergency department (ED). While most SSTI are diagnosed with patient history and physical exam alone, ED clinicians may order CT imaging when they suspect more serious or complicated infections. Patients who inject drugs are thought to be at higher risk for complications from SSTI and may undergo CT imaging more frequently. The objective of this study is to characterize CT utilization when evaluating for SSTI in ED patients particularly in patients with intravenous drug use (IVDU), the frequency of significant and actionable findings from CT imaging, and its impact on subse- quent management and ED operations.

Methods: We performed a retrospective analysis of encounters involving a diagnosis of SSTI in seven EDs across an integrated health system between October 2019 and October 2021. Descriptive statistics were used to assess overall trends, compare CT utilization frequencies, actionable imaging findings, and surgical intervention between patients who inject drugs and those who do not. Multivariable logistic regression was used to analyze patient factors associated with higher likelihood of CT imaging.

Results: There were 4833 ED encounters with an ICD-10 diagnosis of SSTI during the study period, of which 6% involved a documented history of IVDU and 30% resulted in admission. 7% (315/4833) of patients received CT im- aging, and 22% (70/315) of CTs demonstrated evidence of possible deep space or necrotizing infections. Patients with history of IVDU were more likely than patients without IVDU to receive a CT scan (18% vs 6%), have a CT scan with findings suspicious for deep-space or necrotizing infection (4% vs 1%), and undergo surgical drainage in the operating room within 48 h of arrival (5% vs 2%). Male sex, Abnormal vital signs, and history of IVDU were each associated with higher likelihood of CT utilization. Encounters involving CT scans had longer median times to ED disposition than those without CT scans, regardless of whether these encounters resulted in admission (9.0 vs 5.5 h), ED observation (5.5 vs 4.1 h), or discharge (6.8 vs 2.9 h).

Discussion: ED clinicians ordered CT scans in 7% of encounters when evaluating for SSTI, most frequently in pa- tients with abnormal vital signs or a history of IV drug use. Patients with a history of IVDU had higher rates of CT findings suspicious for deep space infections or necrotizing infections and higher rates of incision and drainage procedures in the OR. While CT scans significantly extended time spent in the ED for patients, this appeared jus- tified by the high rate of actionable findings found on imaging, particularly for patients with a history of IVDU.

(C) 2022

  1. Introduction

* Corresponding author.

E-mail address: [email protected] (A.H. Lee).

Skin and soft tissue infections are among the most common diagnoses made in the ED and have been increasing over time [1,2]. They are defined as bacterial infections of the skin and underlying Soft tissues and have a wide spectrum of severity from mild infections

0735-6757/(C) 2022

treated with oral antibiotics to necrotizing infections requiring surgery [3]. Patients who inject drugs (PWID) are especially likely to present to EDs with concern for SSTI, with 64% of PWID reporting at least one episode of SSTI in the previous year [4].

While SSTIs are primarily diagnosed clinically using history and physical exam, laboratory and imaging studies can assist with diag- nosis and treatment in the ED setting [5]. In particular, computed tomography (CT) imaging may be ordered to evaluate for potential deep space or necrotizing infections, which can be associated with significant morbidity and mortality [6] and occur more frequently in the PWID population [4,7].

It remains unclear how often ED clinicians order CT scans for evalu- ation of SSTI and whether Ordering patterns differ substantially in the PWID population. Moreover, it is unclear how often these scans deliver actionable information that alters clinical decision making. Certainly, it is critical to diagnose deep space and necrotizing infections that warrant Surgical consultation, and most CT scans for SSTIs are likely ordered with these conditions in mind [8]. It is also well established that over- ordering of CT scans has downsides: CT scans deliver non-trivial amounts of radiation to patients [9] and are a common operational bot- tleneck in the ED that can exacerbate ED crowding [10]. Given that prior research demonstrates a steady increase in overall CT utilization in EDs, there is also reason to suspect that CT ordering may be increasing in pa- tients with SSTIs as well [11]. It would benefit emergency physicians to better understand patterns and results of CT ordering for SSTI in order to inform appropriate CT utilization in practice.

The objective of this study is therefore to characterize how often ED clinicians order CT imaging in evaluating SSTI, particularly in patients with a documented history of intravenous drug use (IVDU), and how these imaging studies impact Clinical and operational outcomes.

  1. Methods
    1. Study design and setting

We performed a retrospective cohort study of patients presenting to seven EDs in our large healthcare system in the Northeastern United States between October 2019 and October 2021 that were diagnosed with an SSTI. The seven EDs in this health system include two quater- nary care academic medical centers and five community hospitals, with a combined 378,741 ED visits in 2021. The majority of CT imaging was performed with a Siemens 384(2 x 192)-slice scanner and a GE 64- slice scanner The study was reviewed and granted an exemption by the Institutional Review Board (Protocol 2021P003071). The study was per- formed according to STROBE guidelines [12].

Patients were included in this study if they were older than 18 years old and diagnosed with an SSTI in an upper or lower extremity, defined based on ICD-9 and ICD-10 codes, similar to prior work in this field [13]. The sample size using the Kelsey formula [14] was estimated to be at least 1554 patients to compare rates of CT utilization between the IVDU and non-IVDU populations, using alpha = 0.05, power = 0.8, 5% prevalence of IVDU, and an estimated 10% baseline of CT utilization that doubles for patients with IVDU. Patients who were diagnosed with an SSTI involving the face or trunk or with diabetic foot ulcers were excluded from this study, given differences in the subsequent management of these patients.

    1. Outcome measures and data collection

Data was obtained from the institutional Electronic Medical Record (Epic) via Structured Query Language (SQL) query. Demographic vari- ables of interest included patient age, sex, race/ethnicity, and past med- ical history including history of IVDU, diabetes, and HIV. Variables for comorbidities and IVDU were obtained based on inclusion in the Prob- lem List, Past Medical History, or Social History at the time of encounter. The primary outcome variable was whether a CT study was ordered to

evaluate the extremity of interest in the ED. Secondary outcome vari- ables included whether the CT study had positive radiologic findings for deep space infection (defined by deep tissue extension, muscle extension/myositis, Septic arthritis/tenosynovitis, osteomyelitis) or possible necrotizing fasciitis (defined by soft tissue gas tracking along fascial planes and fascial thickening) [15]. Operational vari- ables including time from ED arrival to ED Disposition, rate of admis- sion, and percentage of encounters with documented operative cases within 48 h of ED presentation.

    1. Data and statistical analysis

Radiologic impressions of CT studies dictated by emergency radiol- ogy were extracted from the Epic EMR for review. Two emergency med- icine resident physicians reviewed the impressions and determined whether they would interpret them as being positive for deep infection or NSTI. The kappa statistic for inter-rater reliability between the two resident physicians was 0.71. In cases of disagreement, an attending emergency medicine physician was the tiebreaker. All reviewers were blinded to outcomes. Further statistical analyses were performed in STATA 13 (StataCorp, College Station, TX). Comparisons between con- tinuous variables were calculated using non-parametric median regres- sion and comparisons between categorical variables were calculated using Chi square tests. Multivariate logistic regression was performed to assess the likelihood of CT studies being ordered based on demo- graphic variables including history of IVDU. P values were 2-tailed and considered statistically significant if <0.05.

  1. Results

Between October 2019 and October 2021, there were 4833 ED en- counters within our integrated health system in which a diagnosis of ex- tremity SSTI was made (Table 1). Patients tended to be young (median age 49), male (57%), and Non-Hispanic White (70%). 295 (6%) patients had a documented history of IVDU, 634 (13%) had a documented history of diabetes, and 56 (1%) had a documented history of HIV/AIDS. The majority of patients were discharged (66%), 30% were admitted to the hospital and 4% placed in ED Observation status.

At least one CT scan of an extremity was ordered in 315 (7%) of these ED encounters. Of the 295 ED encounters with a documented history of IVDU, CT scans were ordered in 53 (18%), a significantly higher rate than for patients without IVDU (6%) (p < 0.001) (Table 2). CT scans resulted

Table 1

Patient characteristics.

# of ED Encounters

N 4833

Age: Median (IQR?) 49 (Q1 35, Q3 66)


F 2100(43%)

M 2733 (57%)

Race and Ethnicity

Non-Hispanic White 3400 (70)

Hispanic 655 (14)

Non-Hispanic Black 417(9)

Non-Hispanic Asian 107(2)

Other?? 254(5)


IVDU 295 (6%)

Diabetes 634 (13%)

HIV/AIDS 56 (1%)

ED Disposition

Admitted 1454 (30%)

ED Observation 214 (4%)

Discharged 3165 (66%)

* IQR = interquartile range.

?? Other = Patients with multiple races, Native Americans, Native Hawaiian or Other Pacific Islander, and patients who decline to answer.

Table 2

CT extremity use and findings.

Patients with IVDU

Patients without IVDU


Number of ED Encounters




Number of Encounters where CT was Ordered (%)

53/295 (18%)?

262/4538 (6%)?

315/4833 (7%)

Number of CTs with Positive Superficial Findings (%)

53/53 (100%)

247/262 (94%)

300/315 (95%)

Number of CTs with Positive Deep Findings (%)

13/53 (25%)

57/262 (22%)

70/315 (22%)

OR within 48 h for I&D/Debridement

15/295 (5%)?

105/4538 (2%)?

120/4833 (2%)

* p < 0.05.

Table 3

Logistic regression of patient factors associated with orders for CT imaging.

Patient Factors

Odds Ratio (95% CI)



1.0 (1.8-3.6)


Male Sex

2.1 (1.6-2.8)





Non-Hispanic White

0.4 (0.1-1.9)


Non-Hispanic Black

1.3 (0.8-2.2)


Non-Hispanic Asian

1.1 (0.7-1.6)



0.6 (0.2-1.3)


Fever (>100.4F)?

2.3 (1.5-3.5)


Tachycardia (>100)?

1.8 (1.4-2.3)


Hypotension (Systolic Blood Pressure < 90)?

2.7 (1.9-3.8)


Intravenous Drug Use (IVDU)

2.5 (1.8-3.6)



2.0 (1-4.2)



0.8 (0.6-1.2)


* Binary variables based on worst value obtained while patient is in the ED.

in a similar frequency of positive deep findings in the IVDU population (25%) and non-IVDU population (22%). Overall, patients with IVDU were more than twice as likely to be taken to the OR for incision and drainage or debridement within 48 h of admission (5% for patients with IVDU versus 2% for patients without IVDU, p < 0.01).

A documented history of IVDU was found to independently increase the odds that CT imaging would be ordered (OR 2.5, 95% CI 1.8-3.6). Other variables found to significantly increase the odds of CT imaging were male sex (OR 2.1, 95% CI 1.6-2.8), and abnormal vital signs (fever (OR 2.3, 95% CI 1.5-3.5), tachycardia (OR 1.8, 95% CI 1.4-2.3), hy-

potension (OR 2.7, 95% CI 1.9-3.8)) (Table 3). Poisson regression of fac- tors that increase the odds of positive deep infection findings on CT imaging did not reach significance.

Encounters involving CT scans had significantly longer times from ED arrival to ED Disposition and longer overall ED lengths of stay, regardless of whether the patient was discharged, placed in ED Observa- tion status, or admitted to inpatient services (Table 4). Patients who re- ceived CT scans were also more likely to be admitted to the hospital (77% versus 27%) (p < 0.01) and more likely to undergo an I&D or de- bridement procedure in the OR within 48 h (19% versus 1%) (p < 0.01) than patients who did not undergo CT imaging.

  1. Discussion

ED clinicians in this study managed 93% of ED encounters with a di- agnosis of skin and soft tissue infection without CT imaging. As ex- pected, certain patient factors significantly increased the likelihood of ED clinicians ordering CT imaging, including abnormal vital signs (fever, tachycardia, hypotension) and history of IVDU. Studies describ- ing CT utilization in the ED for SSTI are rare, although one prior study evaluating only upper extremity SSTI found CT imaging utilization of 9.2% in the IVDU and 1.5% in the non-IVDU population [16]. In our study, 17% of the subgroup of ED patients admitted to the hospital re- ceived CT imaging compared with 7% for those admitted to ED Observa- tion and 2% for those discharged, which is comparable to estimates from other studies for the hospitalized population, which have found CT uti- lization rates between 16% [17] and 37% [18].

We found that the IVDU population with SSTI compared to the non- IVDU population has higher rates of CT utilization (18% vs 6%), hospital admission (41% vs 29%), and likelihood of undergoing drainage in the OR (5% vs 2%) within 48 h of presentation. Although we found that CT scans are ordered around three times more often in PWID, the propor- tion of images with findings suspicious for deep space infections or NSTI were similar across groups at around 22%, suggesting the increased ordering frequency for PWID was warranted. Furthermore, our study showed that 19% of patients who received CT scans underwent surgical drainage in the OR as opposed to 1.3% who didn’t receive CT imaging, suggesting that ED clinicians are clinically able to select patients for whom CT imaging is warranted. There was a small minority of patients who received OR management of SSTI despite not receiving initial CT imaging in the ED and this may be due to worsening SSTI despite anti- biotic treatment, inability to tolerate bedside I&D procedures, and sub- sequent assessment by the surgical attending physician after admission to their service. We did not find previous studies examining frequency of finding deep space infections or NSTI on CT imaging, but our other results are consistent with prior literature showing higher rates of CT imaging [16], inpatient admission [19], and OR procedures

[20] for PWID.

As expected, ED encounters for SSTI in which patients underwent CT imaging did have significantly longer lengths-of-stay (LOS) compared to ED encounters without CTs. While clinical complexity may explain

Table 4

operational impact of CT use.

ED Disposition


ED Observation




Patient Encounters




Encounters where CT Ordered

Encounters where CT NOT ordered

where CT Ordered

Encounters where CT NOT ordered

Encounters where CT Ordered

Encounters where CT NOT ordered

Number of Encounters: N (%)

244 (17)

1210 (83)

15 (7)

199 (93)

56 (2)

3109 (98)

Median Time from CT Order Until CT Result:

3.4 (2.2, 5.0)

3.3 (2.1, 4.0)

2.9 (2.2, 4.5)

Hours (IQR?)

Median Time from ED Arrival until ED Disposition: Hours (IQR)

Median ED LOS: Hours (IQR)

9.0 (6.8, 11.7) ??

11.1 (7.8, 21.3) ??

5.5 (3.9, 7.7) ??

6.5 (4.8, 9.4) ??

5.5 (3.7, 11.0) ??

36.4 (19.0, 51.3) ??

4.1 (2.7, 5.7) ??

23.2 (18.3, 29.7) ??

6.8 (5.1,8.3) ??

6.8 (5.1,8.3) ??

2.9 (1.8, 4.3) ??

2.9 (1.8, 4.3) ??

* IQR = interquartile range.

?? p < 0.05.

some of the difference in ED LOS, imaging was likely the primary driver as the median radiology turnaround time from order until initial result was around 3 h which is quantitatively similar to the increase in median time from ED arrival until disposition between patients who receive a CT scan and those who do not, regardless if the patient was admitted or discharged. Prior studies have also demonstrated a significant impact of CT on LOS [21-23]. Although there is increasing adoption of bedside ultrasound and recognition of its reliability in diagnosing superficial cel- lulitis and abscesses [24,25], it is not clear whether it may impact the utilization of CT imaging for assessing deeper and more serious pathol- ogy. Despite the significant impact of CT imaging on ED operational metrics, the high frequency of significant positive findings on CT likely justifies its continued use at current rates for patients with SSTI in both IVDU and non-IVDU populations.

Patients with SSTI in our study had a notably higher admission rate (30%) compared to previous literature, which showed admission rates around 13-15% [26,27], as well as significant length of stay (median 4.1 h). Several factors likely contribute to the high admission rate, in- cluding two community hospitals in our study not having the option of ED Observation as a disposition, the robust system of urgent care clinics in our integrated healthcare system that provides a more conve- nient and lower cost care setting, and the advent of the COVID-19 pan- demic and growth in telehealth visits that discourage in-person visits for low-acuity. The longer ED length of stay is likely also affected by re- cent trends towards increasED wait times and increased ED boarding. While these trends do complicate comparisons with previous literature, they may better reflect how SSTI is currently managed in the ED and should be a subject for further study.

There are multiple limitations to our study. First, identification of IVDU was based on EMR documentation. Given associated stigma and potentially poor access to primary care (where problem lists are cu- rated) in patients with IVDU history, this method likely underestimated the true incidence of IVDU in our population. Second, our study was un- derpowered to detect how individual risk factors may have impacted the odds of positive findings for deep space infections on CT imaging given their relative rarity. Third, some of the CT scans in this study were likely ordered at the request of consulting or admitting services rather than solely at the discretion of ED clinicians, perhaps even to as- sist with operative planning. Fourth, this study focuses on single epi- sodes of SSTI care in the ED and does not evaluate for missed cases of deep infections or long-term complications. Finally, this study focuses on OR management and excludes other procedural treatments such as percutaneous drainage by interventional radiology. There may also be cases where patients decline Operative treatment or hospital admission and this is known to occur at higher rates in patients with IVDU history [28], which may impact some of the descriptive statistics.

Overall, this study demonstrated a relatively low rate of CT utiliza- tion in patients presenting to the ED with SSTI. Patients with IVDU were much more likely to receive CT imaging without a reduction in di- agnostic yield and were also more likely to undergo operative proce- dures within 48 h of admission. Together, this suggests that the higher CT utilization rate observed in patients with IVDU is likely appropriate. Future work might assess how well bedside clinical judgment predicts the need for operative intervention versus CT scans, and how bedside ultrasound may be impacting the need for CT imaging in patients with SSTI.



Summary conflict of interest statement


Prior abstract publication/presentation


Declaration of Competing Interest

All authors report no conflict of interest.


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