Approach

Cellulitis and erysipelas usually manifest as recognizable clinical syndromes. Specific diagnostic tests are not usually necessary. In diagnostic uncertainty, dermatologic consultation may be useful to evaluate for alternative diagnoses.[28]

History

The patient usually presents with an acute onset of a red, painful, hot and swollen area of skin, most often found on a lower extremity.[1][12]​​​​ Constitutional symptoms can include fever, chills, and malaise.

Factors to consider in assessing the patient include:[1][12]​​​​[21]

  • Does the patient have a history of a prior episode of cellulitis? This may be related to lymphatic or venous compromise, which should be evaluated in examination.

  • Is there any disruption of the cutaneous barrier (e.g., ulcers, wounds, dermatoses, tinea pedis interdigitalis)? These may allow introduction of micro-organisms into the skin.

  • Is the patient at risk of infection with MRSA? Infection with MRSA should be considered in patients with recent contact with a healthcare facility or if purulent skin disease is present (or has recently been present) in the patient or their close contacts. Groups that have been identified as having increased risk for infection with community-acquired MRSA include incarcerated individuals, intravenous drug users, military personnel, and athletic team members.[21][29][30]​​

  • Is there a medical history of diabetes? This may predispose the patient to diabetic foot ulcers, which can be complicated by cellulitis.

  • Is the patient immunocompromised? In addition to the organisms causing cellulitis in an immunocompetent patient, immunocompromised patients are more susceptible to infections by aerobic gram-negative bacteria (e.g., Pseudomonas aeruginosa, Helicobacter cinaedi) and nonbacterial pathogens (e.g., Cryptococcus neoformans).

  • Have there been any unusual exposures (e.g., to salt water or fresh water) or is the cellulitis a result of trauma (e.g., a bite)?

Examination

Visual inspection reveals macular erythema with warmth, tenderness, and edema. Cellulitis may have a well-demarcated or more diffuse border.[13][31] The leg is the commonest site. Orange-peel appearance, blistering, superficial bleeding into blisters, petechiae or ecchymoses, and dermal necrosis may occur.[13][32] Lymphangitis and regional lymphadenopathy can also be present.[13][33] Fluctuation on palpation may indicate the presence of an underlying abscess.[34]

Erysipelas is distinguished by its involvement of the more superficial layers of the skin, including lymphatics. Consequently, it distinctively manifests with raised intense erythema and edema as well as borders that are pointedly demarcated from uninvolved skin.[13][23][33] It occurs most commonly on the face and leg.[35] It is almost always caused by group A streptococcus. 

Cellulitis in association with a purulent focus, or recent furunculosis, should prompt consideration of Staphylococcus aureus, including MRSA, as a possible etiology.

Examination may also reveal an identifiable portal of entry (e.g., a wound, ulcer, or signs of tinea infection) and local or regional lymphadenopathy.[36]

In lower-extremity cellulitis, careful examination of the interdigital toe spaces is recommended because treating fissuring, scaling, or maceration may eradicate colonization with pathogens and reduce the incidence of recurrent infection.[13] 

Systemic manifestations are usually mild, but fever, tachycardia, confusion, hypotension, and leukocytosis are sometimes present and may occur hours before the skin abnormalities appear.[13] In cases of orbital and preseptal (periorbital) cellulitis, differentiating between the two is important, as orbital cellulitis is an emergency and may require surgical intervention in addition to antimicrobial therapy. Proptosis and pain or limitation with eye movement suggest orbital disease. See our topic "Periorbital and orbital cellulitis". 

It is important to exclude serious conditions that may be associated with cellulitis and seek urgent senior and/or specialist input as needed. Specifically consider:

  • Septic arthritis (see our topic "Septic arthritis")

  • Osteomyelitis (see our topic "Osteomyelitis")

  • Necrotizing fasciitis (see our topic "Necrotizing fasciitis").

Investigations

Clinical findings alone are usually enough for diagnosis.[32][36]​​

Complete blood count

  • Although nonspecific, most patients have an elevated white cell count.

Purulent focus culture and molecular diagnostic procedures

  • Purulent focus culture should be performed if cellulitis is associated with, or adjacent to, a wound or pustular focus.

  • It may help identify the presence of resistant pathogens such as MRSA and guide antibiotic selection.[37][38]

Blood culture

  • Blood cultures usually have a poor yield in cellulitis; therefore, this test is not sensitive enough for its routine use to be recommended.[13]

  • Blood cultures should be obtained for patients with severe systemic features (such as high fever, tachycardia, or hypotension), and those who are immunocompromised.[13]

Skin aspiration with culture and molecular diagnostic procedures

  • These may identify organisms; however, they are not sensitive or specific enough for their routine use to be recommended.[13][39]

Skin biopsy

  • In cases where the diagnosis is in doubt, skin biopsy is recommended.[40]

  • In addition, in immunocompromised patients or in those who are unresponsive to initial therapy, it can help identify an unusual pathogen.[12]​​​[13]

Samples for culture and sensitivities, if indicated, should be taken before giving antibiotics.[34][41]​​

Venepuncture and phlebotomy: animated demonstration
Venepuncture and phlebotomy: animated demonstration

How to take a venous blood sample from the antecubital fossa using a vacuum needle.

Imaging

Plain radiographs are generally not useful in diagnosing cellulitis. However, they may assist in evaluation of possible subjacent osteomyelitis in cellulitis associated with chronic ulcerations.[43]

If an abscess is suspected, an ultrasound may be useful for confirmation.[43][44][45] An abscess is suspected if tender fluctuance is present on examination or if there is an incomplete response to antimicrobial therapy.

A computed tomography scan is useful in discriminating between orbital and preseptal (periorbital) cellulitis and should be ordered if these diagnoses are being considered.

If necrotizing fasciitis is suspected, magnetic resonance imaging is useful in assisting diagnosis, but it should not delay surgical consultation if necrotizing fasciitis is a consideration.[43][46] Marked pain, especially that judged to be out of proportion to the examination, or a poor response to therapy are factors that suggest necrotizing fasciitis. Hemorrhagic bullae may also be present.

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