Diagnosis
Cellulitis is most often a clinical diagnosis, readily identified in many people by history and physical examination alone, with rapidly spreading areas of cutaneous swelling, redness, and heat, occasionally associated with inflammation of regional lymph nodes. While classically distinguished as a separate entity from erysipelas by spreading more deeply to involve the subcutaneous tissues, many clinicians may classify erysipelas as cellulitis. Both are often treated similarly, but cellulitis associated with furuncles, carbuncles, or abscesses is usually caused by S. aureus, which may affect treatment decisions, especially antibiotic selection. Skin aspiration of nonpurulent cellulitis, usually caused by streptococcal organisms, is rarely helpful for diagnosis, and blood cultures are positive in fewer than 5% of all cases.It is important to evaluate for co-existent abscess, as this finding usually requires surgical drainage as opposed to antibiotic therapy alone. Physicians clinical assessment for abscess may be limited, especially in cases with extensive overlying induration, but use of bedside ultrasonography performed by an experienced practitioner readily discriminates between abscess and cellulitis and may change management in up to 56% of cases. Use of ultrasound for abscess identification may also be indicated in cases of antibiotic failure. Cellulitis has a characteristic "cobblestoned" appearance indicative of subcutaneous edema without a defined hypoechoic, heterogeneous fluid collection that would indicate abscess. Differential diagnosis
Other conditions that may mimic cellulitis include deep vein thrombosis, which can be diagnosed with a compression leg ultrasound, and stasis dermatitis, which is inflammation of the skin from poor blood flow.
Subacute cutaneous lupus erythematosus  is a clinically distinct subset of cases of lupus erythematosus that is most often present in white women aged 15 to 40, consisting of skin lesions that are scaly and evolve as poly-cyclic annular lesions or plaques similar to those of plaque psoriasis.Characteristically the lesions appear in sun-exposed areas such as the vee of the neckline or the forearms, but not the face. It may be brought on by sun-sensitizing medications, but is usually associated with autoimmune disorders such as rheumatoid arthritis and Sjögrens syndrome.Therapy generally involves sun avoidance and protection and topical corticosteroids. Sometimes systemic drug treatment is necessary. Besides corticosteroids other immunosuppressants such as methotrexate are also used.Lesions of SCLE may have an annular (shaped like a ring) configuration, with raised red borders and central clearing. See also
Lupus erythematosus
List of cutaneous conditions
List of human leukocyte antigen alleles associated with cutaneous conditions

References


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