A 45-year-old male with a past medical history of diabetes and hypertension presents with pain and redness to his right anterior thigh. There is a central area of tenderness with surrounding erythema, but there is no fluctuance. The clinician wonders whether this is simple cellulitis or if there is an underlying abscess that requires incision and drainage.
Key clinical questions
What is the accuracy of the physical examination for skin and soft-tissue abscesses?
Traditionally, skin and soft-tissue abscesses have been diagnosed via history and physical exam. In some cases, these infections are obvious and clinically evident with palpable fluctuance or purulent discharge. One study reported that clinical examination alone was 94.7% sensitive and 84.2% specific in patients where the clinicians were confident in their diagnosis . However, in more ambiguous cases, where the clinician was uncertain about the diagnosis, clinical examination alone was only 43.7% sensitive and 42% specific .
What modalities can be used to assess for abscesses?
Computed tomography (CT) is one modality, which has a sensitivity of 76% and specificity of 91.4% . However, it can be time-consuming and exposes patients to radiation and contrast dye. Magnetic resonance imaging (MRI) has also been used as a diagnostic tool with the sensitivity ranging from 77.5% to 90% and a specificity of 88.6% . Similar to CT, MRI is time-consuming and expensive, and it is also not universally available in all emergency departments.
What is the role for ultrasound in diagnosing skin and soft-tissue abscesses?
Point-of-care ultrasound (POCUS) is 94.6% sensitive and 85.4% specific with a positive likelihood ratio of 6.5 and a negative likelihood ratio of 0.06 for the identification of skin and soft-tissue abscesses . Among cases with a high pretest suspicion for abscess or cellulitis, POCUS is 93.5% sensitive and 89.1% specific, whereas among cases that are clinically unclear, POCUS is 91.9% sensitive and 76.9% specific . While the diagnostic accuracy is lower among clinically unclear cases, POCUS still substantially outperforms physical examination in this population [1, 4]. In addition, when added to the initial assessment, POCUS led to a correct change in management in 10.3% of cases and an incorrect change in only 0.7% . Therefore, POCUS has the greatest diagnostic utility in cases that are clinically unclear, while the diagnostic role is much more limited in cases with a high or low pretest probability (i.e., clear evidence of an abscess or uncomplicated cellulitis).
How can ultrasound assist with incision and drainage of skin and soft-tissue abscesses?
POCUS can assist with determining the size of the abscess . It can identify small and superficial abscesses that may not benefit from drainage, as well as the best location to perform the incision and drainage . POCUS can also identify the lateral margins to facilitate the loop drainage technique . Moreover, POCUS can assess for nearby large vasculature, which could complicate or preclude bedside incision and drainage. In addition, it can ensure that the lesion is not a vascular abnormality, like an aneurysm or pseudoaneurysm. When the diagnosis is less clear, color Doppler could also help delineate a cyst (e.g., smooth, clearly defined border without hyperemia) or lymph node (e.g., well-circumscribed border with internal hyperemia) from an abscess (e.g., more serpiginous border with surrounding hyperemia). POCUS can also evaluate to ensure that adequate drainage was performed, as well as for the presence of deeper abscesses which may require more extensive incision and drainage. Finally, it can determine which abscesses are too deep or complicated (e.g., proximity to vasculature, necrotizing fasciitis) and require drainage in the operating room or by interventional radiology.
How is the ultrasound examination for skin and soft-tissue abscesses performed?
Place a linear transducer over the area of cellulitis or suspected abscess. While looking at the subcutaneous fat and fascial layers overlying the muscle below, slide along both longitudinal and transverse planes to determine the extent of the infection as well as to identify any organized pockets of hypoechoic fluid (i.e., abscess). For uncomplicated cellulitis, look for fluid coursing between fat lobules referred to as cobblestoning. Cobblestoning represents reactive edema within the tissue and does not represent drainable fluid. Abscesses will appear in the subcutaneous fat and may be simple (e.g., hypoechoic, saccular) or complex (e.g., heterogenous, septations) in appearance (Fig. 1). When unsure about the nature or size of a collection, compress the lesion to watch for flow or movement of contents and determine the extent of the complex collection . If an abscess is confirmed, determine its size in both long and transverse plane, and identify the optimal incision point, balancing minimal depth from skin surface with the largest pocket of fluid. Finally, use color Doppler to assess for hyperemia and nearby vasculature.
What is the role of ultrasound for diagnosing necrotizing fasciitis?
POCUS can help quickly identify and expedite surgical treatment in patients with suspected necrotizing fasciitis. The findings are best recalled with the STAFF mnemonic: subcutaneous thickening, air (with associated artifacts), and fascial fluid . Among these, fascial fluid has the greatest diagnostic ability, but varies depending upon the amount of fluid present. One study found that 2 mm of fascial fluid was 75% sensitive and 70% specific . However, the sensitivity decreased to 42% and specificity increased to 94% when 4 mm of fascial fluid was present .
Another study utilizing diffuse subcutaneous thickening combined with 4 mm of fascial fluid reported that POCUS was 88% sensitive and 93% specific . While POCUS is not sufficiently sensitive to rule out a necrotizing infection, the findings are fairly specific and can be helpful in mobilizing resources in this time-sensitive condition. In addition, POCUS can be performed at the patient’s bedside and can be easily repeated with changes in the patient’s clinical condition.
The clinician utilizes POCUS to evaluate the area and identifies a 3 cm-by-4 cm hypoechoic fluid collection 2 cm below the skin surface. There is surrounding hyperemia and no nearby large vasculature. The clinician successfully performs an incision and drainage. The patient is discharged and has an uneventful recovery.
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Conflict of interest
The authors declare that they have no conflict of interest.
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Gottlieb, M., Sundaram, T., Kim, D.J. et al. Just the facts: point-of-care ultrasound for skin and soft-tissue abscesses. Can J Emerg Med 23, 597–600 (2021). https://doi.org/10.1007/s43678-021-00132-9