Introduction

Point-of-care ultrasound is a useful tool for evaluating patients with skin or soft tissue complaints. It can accurately differentiate between cellulitis and abscess, aid in the diagnosis of necrotizing soft tissue infections and assists in foreign body localization. In addition, the use of point-of-care ultrasound has been shown to decrease wait times and improve patient satisfaction with emergency department care [1,2].

Why ultrasound?

Patient presentations for soft tissue infections are common. Over the last 40 years, the number of children presenting to emergency departments with skin and soft tissue infections has doubled [3]. Ultrasound is invaluable when examining soft tissue and has much better test characteristics than physician judgement and physical exam combined for differentiating between abscess and cellulitis [4]. With the growing prevalence of MRSA and the need for different management plans for cellulitis versus abscess, being able to differentiate between the two entities is important for frontline providers.

PoCUS has been shown to have a sensitivity of 98%, specificity of 88%, positive predictive value of 93% and negative predictive value of 97% for the diagnosis of cellulitis this is significantly better than clinical exam alone which has a sensitivity of 86%, specificity of 70%, positive predictive value of 81% and negative predictive value of 77% [4]. Additional studies have shown that ultrasound can be used to identify abscesses with a positive likelihood ratio of 5.5 and negative likelihood ratio of 0.04 [5]. Test characteristics are similar among both adult and pediatric patients [6,7]. A prospective cohort study of adult patients presenting to an urban ED with clinical soft-tissue infection (but no obvious abscess) found that 48% of patients who were clinically thought to be cellulitis alone, were found to have abscesses requiring incision and drainage when examined with ultrasound [8]. A similar study in pediatric patients found the use of PoCUS changed management in 22% of patients [9]. Point-of-care ultrasound also helps differentiate abscess from other pathology, thereby decreasing the chance of complications arising from incision and drainage. In complicated cases, the completeness of abscess drainage can be assessed, reducing the rate of treatment failure.

PoCUS can also aid in differentiating necrotizing fasciitis from other infections. When compared to biopsy findings, PoCUS demonstrating diffuse subcutaneous tissue thickening and fluid accumulation along the fascial plane has a sensitivity of 88% and specificity of 93% for necrotizing fasciitis [10].

Table 1:

Diagnostic Accuracy of Ultrasound in Soft Tissue Infection
	Cellulitis	Abscess	Necrotizing Infection
Sensitivity	98%	97% (CI: 94-98%)	88.2% (CI: 63.6-98.5%)
Specificity	88%	83% (CI: 75-88%)	93.3% (CI: 81.7-98.6)
Positive LR	—	5.5 (CI: 3.7-8.2)	—
Negative LR	—	0.04 (CI: 0.02-0.08)	—
Positive PV	93%	—	83.3% (CI: 84.5-99.4%)
Negative PV	97%	—	95.4% (CI: 82.2-97.3%)

 

Ultrasound is also a useful tool for identification of soft tissue foreign bodies. Plain film radiography can only identify radiopaque objects, while ultrasound is able to identify both radiopaque and non-radiopaque foreign bodies. In addition, ultrasound can be used procedurally to guide foreign body removal and to confirm the success of the procedure. Ultrasound is superior to x-ray for radiolucent foreign bodies, with a sensitivity of 87% and specificity of 97% for wooden foreign bodies larger than 2.5 mm in length [11] However, sensitivity and specificity decreases compared to x-ray for radiopaque foreign bodies. A good general practice when faced with a patient concerned about a soft tissue foreign body is to start with bedside ultrasound and then move to further imaging if the result of the ultrasound is negative.

Table 2:

Diagnostic Accuracy of Ultrasound in Retained Soft Tissue Foreign Bodies
	Foreign Body	Radiolucent Foreign Body
Sensitivity	72% (95% CI: 57-83%)	96.7% (95% CI: 90.0-99.1%)
Specificity	92% (95% CI: 88-95%)	84.2% (95% CI: 72.6-92.1%)
Positive LR	3.2 (95% CI: 2.1-5.1)	5.5 (CI: 3.7-8.2)
Negative LR	0.11 (95% CI: 0.08-0.16)	0.04 (CI: 0.02-0.08)

[12]