Necrotizing Fasciitis and Fascial Fluid

By Michael Prats

Nec fasc

The Relationship Between Fluid Accumulation in Ultrasonography and the Diagnosis and Prognosis of Patients with Necrotizing Fasciitis

Ultrasound Med Biol July 2019 - Pubmed Link

Take Home Points

1. Based on current limited data, point of care ultrasound findings of necrotizing fasciitis are not sufficient to rule in or out the diagnosis.

2. Patients with necrotizing fasciitis and fascial fluid had longer hospital length of stay.

3. Sonographic fluid along the fascial plane may be sensitive for severe disease but more data is needed.


Necrotizing fasciitis is no joke. This is the worst case scenario for skin and soft tissue infection, and people that have it die at an alarmingly high rate. As soon as the diagnosis is made - early surgical intervention can be indicated. The diagnosis can be made clinically (or operatively), but sometimes imaging is needed. Instead of waiting around for CT or MRI - ultrasound could be a way faster way to confirm the diagnosis. Some people have been already using ultrasound to diagnose this deadly disease, most famously the STAFF exam has been developed to remember the sonographic signs. However, because this is not a super common problem and ultrasound is not routinely used for the diagnosis - we have little idea how accurate it is for this disease. There was one small study from about 15 years ago that showed a specificity in the low 90’s, but we need more evidence. And that brings us to the study at hand - how accurate is ultrasound for the diagnosis of necrotizing fasciitis? And furthermore - can it be prognostic?

Check out The Evidence Atlas - Soft Tisse/MSK for a review of prior evidence on point of care ultrasound for Soft Tissue and musculoskeletal.


How does the ultrasonographic finding of fluid along the deep fascia relate to the diagnosis of NF?

How does this finding relate to the prognosis?


Single academic center in Taiwan


  • Adults

  • Suspected NF of the limbs

  • Between 2/2015 and 11/2016


  • Previously received antibiotics or debridement

  • Lesions on trunk

  • Presented outside of hours 7am to midnight

  • Bilateral sonographic signs of fascial fluid accumulation


Retrospective convenience cohort

Patients were suspected of necrotizing fasciitis based on symptoms and exam (pain out of proportion, rapid progression, crepitus, skin bullae, necrosis, ecchmyosis)

All patients received ultrasonography within 1 hour of arrival to emergency department

Orthopedic surgery consulted after lab work

Data collected until discharged from ward including number of operations, LOS, mortality.

The standard for comparison was diagnosis of NF based on pathology. If did not have surgery or had negative pathology, classified as non-NF.

Who did the ultrasounds?

Three experienced emergency physicians

8 hour basic and soft-tissue ultrasound training prior to study

The Scan

Linear transducer


No protocol specified. Presumably they just imaged over the affected area.

Looking for:

  • Irregular or thickened fascia

  • Emphysema

  • Fluid accumulation along deep fascia

  • Depth of fluid

  • Compared to contralateral side

Learn how to look for Nec Fasc from 5 Minute Sono!


Check out what soft tissue pathology looks like at the POCUS Atlas!



N = 95 patients

  • 48 patients (50.1%) with NF

  • 27 (28.4%) female

  • NF group has more diabetes, cirrhosis, alcohol use disorder, longer hospital length of stay

  • 4 patients (8.3%) died in NF group, 2 (4.3%) died in non-NF group. Not statistically significant different.

Primary Outcome - Accuracy of Fascial Fluid on POCUS for NF

AUC 0.774 (0.679-0.869)

Best cut off to diagnosis NF was 2 mm of fluid accumulation

Sensitivity 75%

Specificity 70.2%


Patients with NF + fluid accumulation had longer length of stay 39 day average vs 23 day average. Patients with NF had longer average length of stay than patients with non-NF (36 days vs 8 days)

In patients with NF, those who had sonographic fascial fluid did not have a statistically significant difference in the median number of operations.

Other Findings

Small number of patients had subcutaneous emphysema (3/48), but all of these had NF

Fluid accumulation was 100% sensitivity for patients who require amputation or died.

Between the three study operators - there was 100% inter-rater reliability on 10 patients in ED with soft tissue infections.

Fluid accumulation occurred in 55.3% of non-NF and irregular or thickened fascia occurred in 44.7% of non-NF.

If you used cut off of 1 mm - 86.5% sensitive. If you used cut off of 5 mm - 97.9% specific.


Usual things: Small study, trained operators. Although their amount of training was not crazy, they did have excellent interrater reliability (100%!).

Considering external validity - this Southern Taiwan population may not have the same bacteria or characteristics as your patients. Seems to be a high incidence of NF at this place. Over the 21 months of enrollment there were 48 cases, so about 2 per month. In addition, this was 50% of the patient’s they suspected could have NF! That may also be higher than many are used to.

Two types of bias - first the usual problem of retrospective studies (many unaccounted for biases in the population) and secondly, being that this was a convenience sample only enrolled during certain hours of the day further influences this population.

Exclusion criteria - why were patients with suspected or actual truncal NF excluded? Were potential cases of Fournier’s gangrene excluded? It would be nice to see the breakdown of patients that were excluded and understand the rationale behind this.


The authors used a 4-12MHz probe for all the scans. We are looking for an optimal FF depth of 2mm. In the hands of novice users and/or using lower frequency linear array probes, is it reasonable to expect to measure this accurately? Also, for a patient with an acutely painful limb, it would be technically difficult to measure the size of a stripe of FF in the diseased tissues. This may not be practical for non-expert users. May be similar to “how big is the CBD scenario” that many struggle with.

How to use this information? First, how not to use it. Don’t use ultrasound to “rule out” the diagnosis if it appears that way clinically - get them to the operating room! This is another one of those scenarios where you shouldn’t let your passion for ultrasound get in the way for patient care. If they obvious have nec fasc - don’t waste time getting any imaging. This modality might be best used for patient’s who do not have a clear diagnosis, they look well, but you feel like you need to rule out the disease. You plan to obtain a CT, but want to get some information earlier. Keeping in mind the poor test characteristics, perhaps if you see concerning ultrasound findings such as air or a large amount of fascial fluid - this could help you expedite surgical consultation and management. It is nice that the authors could conclude that NF patients with FF had a longer LOS than those patients without but not sure how useful this would be in the acute setting.

Take Home Points

1. Based on current limited data, point of care ultrasound findings of necrotizing fasciitis are not sufficient to rule in or out the diagnosis.

2. Patients with necrotizing fasciitis and fascial fluid had longer hospital length of stay.

3. Sonographic fluid along the fascial plane may be sensitive for severe disease but more data is needed.

More Great FOAMed on this Topic

SonoStuff - POCUS for Soft Tissue

Emergency Medicine Cases - Necrotizing Fasciitis

Our score

3 Probes

Expert Reviewer for this Post


Cian McDermott FRCEM FACEM CCPU @cianmcdermott

Emergency Physician, Mater Misericordiae University Hospital, Dublin, Ireland

Reviewer's Comments

Ok I had help with this review! My disclaimer : Shirley, my partner, is a plastic surgeon and has seen her fair share of NF. We bounced around questions about this study on a road-trip in Ireland recently. Furthermore she has worked in Chang Gung hospital in Taiwan for 6 weeks in 2018 thus has 1st hand knowledge of the centre where the authors work. She says (I know you will like this Mike!) - “NF is a minor incident with a major outcome” She also says - “NF needs a knife not an ultrasound” and that correlates with what you have said in the discussion. At best POCUS for NF is useful to expedite surgical consult - the test characteristics probably do not allow us to definitively say anything else. The LRINEC risk classification score is a clinical decision instrument devised by a Singaporean plastic surgeon. It is used by some to help in diagnosis, but it is controversial due to poor performance in validation studies. Could POCUS + LRINEC be the holy grail for early diagnosis of NF? Maybe. In short, I have concerns about the internal and external validity of this paper. It’s a noble attempt to figure out the best cut-off for FF in the tissues but ultimately I’m afraid that this may be flawed. The authors may have missed an opportunity to co-relate LRINEC scores and POCUS findings for a more clinically useful algorithm at the bedside.

Cite this post as

Michael Prats. Necrotizing Fasciitis and Fascial Fluid. Ultrasound G.E.L. Podcast Blog. Published on August 19, 2019. Accessed on December 05, 2020. Available at
Published on 08/19/19 06:00 AM
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