Sometimes things aren’t as they seem.  As an attending I often hear from a resident “Hey got an easy one for ya, 50 y/o diabetic with bilateral cellulitis vanc given, I’ll get some admit labs” followed by a precipitous mic drop.

I was told once that a good attending “makes a difficult case simple and a simple case difficult” I believe that more and more these days. So is a simple case of cellulitis really that simple? Well in a study by Weng in 2016, 30% of 259 pts were misdiagnosed with cellulitis. A similar study by Li also 30 percent of 116 patients were incorrectly diagnosed as cellulitis” in a study by Li in 116 patients. Thirty percent is a large number, but does it surprise me? Not really.  I have heard so many times a patient is getting admitted for “bilateral cellulitis”.  So you are telling me there are two skin infections starting at the feet and racing to the groin like two trains approaching each other??? If you think about it kind of doesn’t make sense that this would exist much. Maybe in a very immune-compromised patient it can occur but otherwise the patient should be really sick. If I really think about it in IVDU patients with skin abscess all over even they don’t have bilateral cellulitis! In fact every case report of bilateral cellulitis I found was actually on how “bilateral cellulitis” was the WRONG diagnosis.

So if its not cellulitis then what is it (we will get back to the bilateral part in a bit)?


I asked an intern what is his approach to a patient differential diagnosis. “Worst first” he said. I chuckled a bit since I appreciate alliteration. But in an overall way he is on the right track so on that note lets start off with the worst: Necrotizing fasciitis (NF). However, this will not be a lengthy discussion because there are so many good resources on NF. Instead I want to focus on some pearls for this diagnosis. To start one thing that makes me feel better is this quote in Clinical infections disease by Anaya “Establishing the diagnosis of necrotizing soft tissue infections is not easy”. Thanks for that, what no clinical correlation recommended? The typical findings of blisters/bullae, crepitus, gas, fever, tachycardia, hypotension, and shock have a low sensitivity of only 10-40%.  Well is there anything that can help us? When all else fails examine the patient. We need to see if the area is necrotic. So, one helpful trick is the “finger test”in the almost brilliantly named BMJ article “Necrotizing Fasciitis: Always used the finger” (which I would have said give them the finger but the Brits always have to be proper). Here a test incision is made in the suspected area of approximately 2 cm. A positive test would be characterized by the absence of normal blood flow, dirty’ dishwater’ colored fluid and discoloration of the fat. Then a rapid finger sweep at the level of the fascia can be carried out. If the tissues dissect with minimal resistance this again favors the diagnosis.



The simplest way to think about this would be to say there is cellulitis and then there is everything else. So what are some pearls for the diagnosis of these tricksters?

  1. Cellulitis is rarely bilateral! Fact!
    1. Corollary: Vascular dermatitis is usually bilateral
  2. Elevate the leg when you examine it! Dependent redness is often mistaken for cellulitis, but erythema promptly disappears after elevating the leg at the bedside. Dependent redness is often asymptomatic, but can be associated with rest pain from arterial insufficiency.
  3. Stasis dermatitis (AKA venous eczema) is the most common mimic of cellulitis.
  4. Superficial thrombophlebitis can appear similar to lymphangitis however DVT of the lower limb rarely causes cutaneous erythema EXCEPT in the proximal thigh, where the femoral vein lies just below the skin surface
  5. Cellulitis is not typically itchy, its painful
    1. Corollary: IF the area is insensate worry about Nec Fasc!
  6. Malignancy affecting the lymphatics of the lower extremities can closely mimic cellulitis!
  7. Failure of  antibiotics should make you think twice about the diagnosis of cellulitis
  8. Cellulitis should have inguinal lymphadenopathy
  9. Confluent Erythema Nodosum can have a similar appearance but will have a different feel since it is a panniculitis
    1. Corollary: Cellulitis should be smooth and not be made up of firm nodules!
  10. Erythema Multiform can have a similar appearance and mistaken for allergic reaction or cellulitis.


Stasis Dermatitis (aka Venous Eczema):

  • Ill-defined, bilateral, pitting edema of the lower extremities, typically with erythema, hyperpigmentation, serous drainage, and su- perficial desquamation
  • Chronic venous insufficiency causes micro- vascular changes and microthrombi leading to acute cutaneous inflammation
  • ill-defined erythematous plaque with overlying pigment changes and super- ficial desquamation, as well as nonpitting edema


  • Necrosis, and fibrosis of the subcutaneous fat, especially in women
  • It usually develops much more slowly than cellulitis—over weeks to month
  • A sclerosing panniculitis classically described as an “inverted champagne bottle” or “inverted bowling pin” appearance of the leg, ie, the diameter of the leg is sharply narrowed directly below the calf
  • There is an acute and a chronic phase. The acute phase is characterized by inflammation and erythema, and the chronic phase is characterized by fibrosis
  • The acute phase can be difficult to differentiate from cellulitis. venous insufficiency, cutaneous changes of stasis dermatitis, and the absence of systemic symptoms all point to lipodermatosclerosis.

Contact Dermatitis

  • The lesion is a painful, nonpruritic, well-demarcated, erythematous, weeping plaque with scattered vesicles at the periphery, as well as superficial desquamation and scaling.
  • Ask about recent changes in medications, soaps, and laundry detergents, new hobbies, or recent surgeries. The involved site is often confined to the area where the allergen contacted the skin


  • Localized edema of an affected extremity, with induration, erythema, and secondary cutaneous changes such as hyperkeratosis, dyspigmentation, and wart-like architecture
  • Has the patient undergone lymph node dissection? Has the patient had an injury in the affected leg? Lymphedema is overwhelmingly unilateral and nonpitting, and is often seen in overweight people

Eosinophilic cellulitis, or Wells syndrome

  • It is a recurrent hypersensitivity reaction to a drug, to a vaccine, or to an insect bite, or to a viral or fungal infection that pre- sents on the extremities as localized erythema, edema, and induration with sharp borders and a green or gray hue
  • Patients tend to report itching and burning that precedes the
  • onset of plaques the complete blood count typically shows a transient hypereosinophilia


  • is a rare disorder that causes episodes of heat, redness, and burning discomfort provoked by heat and dependency and relieved by elevation and cooling of the extremity
  • Primary occurs more common in women than men
  • Can be a marker of systemic disease associated with: myeloproliferative neoplasms of polycythemia vera, essential thrombocythemia, and chronic myelogenous leukemia.
  • Age of onset is typically in the forties and fifties.
  • The symptoms are nearly always intermittent, with episodes usually lasting from a few minutes to a few hours, but occasionally lasting for several days
  • They are typically provoked or worsened by limb dependency, exercise, and heat and alleviated by the opposite of these


  • The onset of an attack is abrupt, causing severe pain, tenderness, erythema, swelling, and warmth over the affected joint.
  • Evolves rapidly, reaching its maximal intensity within 6 to 12 hours.
  • Commonly expands a considerable distance beyond the joint itself, producing extensive cutaneous inflammation that may strongly resemble cellulitis.
  • Fever occasionally occurs,
  • Careful examination, , usually indicates that the origin of the inflammation is clearly in the synovial space, rather than the soft tissues
  • Movement of the affected joint and pressure over it produce exquisite pain that is less intense with compressing adjacent inflamed, non- articular tissue
  • Don’t order a serum uric acid! It doesn’t help!


  1. Weng QY, Raff AB, Cohen JM, Gunasekera N, et al. Costs and consequences associated with misdiagnosed lower extremity cellulitis. JAMA Dermatol.
  2. Li et al. Outcomes of Early Dermatology Consultation for Inpatients Diagnosed With Cellulitis. JAMA Dermatol. 2018 May 1;154(5):537-543.
  3. Anaya et al. Necrotizing Soft-Tissue Infection: Diagnosis and Management. Clinical Infectious Diseases, Vol. 44, No. 5 (Mar. 1, 2007), pp. 705-710
  4. Necrotizing Fasciitis: Always use the finger. BMJ 2005;330:830
  5. Lower limb cellulitis and its mimics Part II. Conditions that simulate lower limb cellulitis. J Am Acad Dermatol 2012;67:177.e1-9.
  6. Keller EC. cellulitis mimics. Cleveland Clinic Journal Of Medicine. 79: 8 2012 547- 552

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