Cellulitis over/underdiagnosis


A patient presents with a red swollen leg. Discharge with oral antibiotics or admit for IV antibiotics? Not so fast.


An recent article highlights the error rate of “cellulitis” diagnoses: Costs and consequences associated with misdiagnosed lower extremity cellulitis. JAMA Dermatol 2016 Nov 2, E1-6. In this retrospective cross-sectional study of patients admitted from the ED of a single hospital with a diagnosis of cellulitis, a third had a misdiagnosis—the majority of whom did not require either antibiotics or hospital admission.


A pair of review articles from the Journal of the American Academy of Dermatology summarize the diagnosis and mimics of lower limb cellulitis. Lower limb cellulitis and its mimics, part I: lower limb cellulitis. J of Amer Acad Derm 2012, 67(2), 163-177


Cellulitis: superficial spreading skin infection without underlying pus

Risk factor: edema, venous insufficiency, obesity, trauma, ulceration, dermatologic disorder

Symptom/signs: rubor, tumor, calor, dolor

-redness: well demarcated, irregular, persists on elevation, +/- lymphangitis or petechiae

-edema: raised, skin may be tight or dimpled, +/- bullae

-warmth, but patient often afebrile with normal WBC

-pain/tender mild


Treatment: 5 days cephalexin + ibuprofen + elevation +/- prednisone

Unusual causes: immunocompromise (gram negative, fungal), fresh water (aeromonas), salt water (vibrio), animal bite (pasteurella)



The second review article lists a number of cellulitis mimics. Lower limb cellulitis and its mimics, part II: conditions that simulate lower limb cellulitis. J of Amer Acad Derm 2012, 67(2), 177-85.

1) Dependent rubor, caused by occlusive vascular disease: disappears on elevation


2) Venous disease

-venous insufficiency: hyperpigmentation, venous ulcer (medial malleolus): stasis dermatitis (weeping, crusting, scaling, pruritis) or lipodermatosclerosis (tender red plaque over weeks/months)



-DVT not red unless femoral vein in inguinal fold

-superficial thrombophlebitis: palpable tender cord

3) Eczema/dermatitis

-asteototic eczema (eczema craquelé): dry, scaling, fissuring, pruritis


-irritant contact dermatitis: burning well-demarcated after exposure


-allergic contact dermatitis: pruritic/blistering eruption after exposure

4) Gout: sudden severe pain/red/warm joint (great toe, knee), pain on ROM


5) Carcinoma erysipelatoides: subacute cancer invasion of lymphatics, red hot plaque


6) Lymphedema: edema and hyperpigmentation from lymphatic injury (cancer, trauma, surgery, radiation, venous insufficiency)



In addition to overdiagnosing cellulitis, there’s also the possible error of underdiagnosis—missing a more serious or complicated infection.


1) Necrotizing Fasciitis

NF is the most feared skin infection. A classic emergency diagnosis, it is rare, life-threatening, and has the best chance of treatment when identified early—when it is most difficult to diagnose.


Briefly, its features are

Risk factors:

-chronic disease: diabetes, PVD, liver/renal failure, immonocompromised

-recent trauma: surgery, perfed abdomen, injury, IVDU, burn, abrasion

Etiology: type I polymicrobial (most common), type II strep, type III clostridium or gram-negative, type IV fungal


-severe infection: rapid expansion, pain out of proportion

-spread through fascial plane: edema/tender beyond erythematous border

-gas-forming bacteria: crepitus

-tissue destruction: cutaneous anesthesia, gangrene

-systemic features: septic shock, multiorgan dysfunction


Labs: there’s been an attempt to use labs to rule in/out NF: Lab Risk Indicator for NF (LRINEC): CRP>150 = 4 points; WBC > 15 = 1 point, >25 = 2 points; Hgb <135 = 1 point, < 110 = 2 points; sodium < 135 = 2 points; Creatinine > 141 = 2 points; Glucose >10 = 1 point

-interpretation: > 6 has PPV 92% and NPV 96%; but 10% of patients < 6 points had NF

-recent review of 80 patients found LRINEC had a sensitivity of only 77%

(Inadequate sensitivity of laboratory risk indicator to rule out necrotizing fasciitis in the emergency department. West J Emerg Med 2016 May;17(3):333-6.)

Imaging: Xray may show gas but is poorly sensitive for NF

CT or MRI and superior but are not 100% and can delay surgery


1) fluid resuscitation and broad spectrum antibiotics

2) and surgical debridement without delay

3) hyperbaric oxygen? No clear role from Cochrane review

(adjunctive hyperbaric oxygen for necrotizing fasciitis.

Cochrane Database Syst Rev 2015 Jan 15)


A recent review from a tertiary pediatric centre demonstrated the challenges of making an early diagnosis: A 10-year review of necrotizing fasciitis in the pediatric population: delays to diagnosis and management. Clin Pediatr 2016 Sept 23, 1-7.

-risk: 33% recent trauma, 25% recent varicella, 17% recent other infection

-presentation: <50% triaged as urgent, <50% febrile, 25% had WBC>25

-time to antibiotics 2.6 hours

-time to surgical consult 4.6 hours

-time to debridement 22 hours


There is increasing recognition of the role of ultrasound for early diagnosis. The following images are taken from helpful post on sonostuff.

POCUS of normal tissue will show well organized tissue planes:


Simple cellulitis will show cobblestoning of the hypodermis due to edema.


But with NF there can be additional findings—summarized the acronym STAFF (Diagnosis of necrotizing fasciitis with bedside ultrasound: the STAFF exam. West J Emerg Med 2014 Feb;15(1):111-3.):

Subcutaneous Thickening

Air: dirty shadow, reverberation artifact


Fascial Fluid < 4mm



The last image was taken from  a prospective observational review of 62 patients with severe cellulitis of which a quarter had NF, where ultrasound was found to have a sensitivity of 88% and a specificity of 93%: Ultrasound screening of clinically-suspected necrotizing fasciitis. Acad Emerg Med 2002 Dec;9(12)::1448-51. This may even be superior than advanced imaging—as demonstrated in this case report, where NF diagnosed with ultrasound had surgery delayed by lack of findings on CT. Point-of-care ultrasound diagnosis of necrotizing fasciitis missed by computed tomography and magnetic resonance imaging. J Emerg Med 2014 Aug;47(2):172-5



2) Abscess

We are taught to clinically differentiate abscess from cellulitis based on the presence or absence of fluctuance. But this sign is not very accurate. In 2006 it was shown to change the management in half of patients presenting to the ED with cellulitis, including showing the need for drainage when none was thought needed, and showing that I&D was not needed when it was originally planned: The effect of soft-tissue ultrasound on the management of cellulitis in the emergency department. Acad Emerg Med 2006;Apr;13(4):384-8.

While this study had sonologists performing the ultrasound, a follow-up study of emergency physicians attending a brief training video found that use of POCUS improved accuracy of diagnosis over physical exam alone: sensitivity increased 86% to 98%, specificity increased 70% to 88%. ABSCESS: applied beside sonogrpahy for convenient evaluation of superficial soft tissue infections. Acad Emerg Med 2005 Jul;12(7):601-6.

Since then there have been more studies, which have recently been reviewed: In patients presenting to the emergency department with skin and soft tissue infections what is the diagnostic accuracy of point-of-care ultrasonography for the diagnosis of abscess compared to the current standard of care? A systematic review and meta-analysis. BMJJ Open 2017 10;7(1). Reviewing eight good to excellent articles, the combined test characteristics for the POCUS diagnosis of abscess: 96% sensitivity, 82% specificity, LR+ 5.6, LR- 0.05


POCUS of an abscess shows a mass with internal echoes.


DDx:    -soft tissue mass: pressure gives “swirl sign” in abscess


-pseudoaneurysm: Doppler flow

-hernia: bowel wall

-lymph node: oval structure like mini-kidney


POCUS also helps guide incision by indicating the area of maximal fluid, and highlighting with Doppler the vessels to avoid



3) foreign body

Delayed diagnosis of foreign bodies can be a source of abscess or “recurring cellulitis—and are a common source of malpractice (Reducing risk in emergency department wound management. Emerg Med Clin of North America 2007, 25(1):189-201). Even if suspected, Xray only shows 80% of all foreign bodies and misses many organic foreign bodies like wooden splinters. CT is superior to Xray but it too can miss small organic foreign bodies. But POCUS is fast and accurate showing hyperechoic focus with acoustic shadow: Non-opaque soft tissue foreign body: sonographic findings. BMC Med Imaging 2011;11:9. In a 2015 review and meta-analysis, pooled characteristics from 17 studies fond ultrasound to have a sensitivity of 72% and specificity of 92% (Diagnostic accuracy of ultrasonography in retained soft tissue foreign bodies: a systematic review and meta-analysis. Acad Emerg Med 2015 Jul;2297):777-87.)




Before diagnosing cellulitis

1. consider overdiagnosis: is it non-infectious?

-skin: eczema/dermatitis

-joint: gout

-chronic arterial: dependent rubor

-chronic venous: stasis

-acute venous: DVT, thrombophlebitis


2. consider underdiagnosis: is it a complicated infection?

-NF: feel for crepitus, pain out of proportion, anesthesia, edema beyond. POCUS: Subcutaenous Thick, Air, Fascial Fluid. Get a surgery consult if you’re concerned, regardless of labs/imaging, and give broad-spectrum antibiotics early

-abscess: POCUS to diagnose and guide I&D

-foreign body: POCUS for foreign body



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