| Etiology | Distinguishing features |
| Infectious |
| Erysipelas | - Bright red appearance, most commonly on malar region of face
- Borders are raised and distinct
|
| Necrotizing soft tissue infection (eg, necrotizing fasciitis)* | - Dusky skin appearance or woody induration
- Pain out of proportion to exam; tenderness may extend beyond the area of visible skin inflammation
- Rapid progression with severe sepsis or shock
- Crepitus and/or bullae in some cases
|
| Toxic shock syndrome* | - Sustained hypotension
- Signs of organ failure (eg, acute kidney injury, thrombocytopenia)
|
| Clostridial myonecrosis (gas gangrene)* | - Rapidly progressive severe muscle pain
- Crepitus and/or bullae in some cases
|
| Skin abscess | - Tender, erythematous, fluctuant subcutaneous nodule
|
| Pyomyositis* | |
| Vascular graft infection* | - Cellulitis overlying a vascular graft
- Bloodstream infection
|
| Mycotic aneurysm* | |
| Native or prosthetic joint infection* | - Erythema overlying a joint
- Severe pain with joint manipulation, or inability to bear weight due to pain
- Prosthetic joint infection may have less severe pain
|
| Acute osteomyelitis | - Erythema, edema, and point tenderness overlying bone
|
| Erythema migrans | - Nontender, slowly progressive round or oval erythematous patch
- May have central clearing
- Patient may recall tick bite
|
| Erythrasma | - Brown- or orange-hued erythema
- Distinct borders
- Located in intertriginous areas
|
| Herpes zoster | - Vesicles or crusted lesions in dermatomal pattern
|
| Vascular |
| Venous stasis dermatitis | - Chronic scaly plaques with acute erythematous exacerbations
- Usually bilateral
- May be pruritic
- Often involves the medial ankle
|
| Lipodermatosclerosis (panniculitis from chronic venous stasis) | - Firm, pigmented, painful induration
- Often begins on medial ankle
- Can be unilateral or bilateral
|
| Lymphedema | - Chronic extremity edema
- May have pitting or firm induration
- Can be unilateral or bilateral
|
| Deep venous thrombosis* | - Unilateral extremity edema
- Erythema, warmth, tenderness, and rate of progression less impressive compared with cellulitis
- May have low-grade fever
|
| Hematoma | - Erythematous, orange, or violaceous hue
- Often associated with local trauma
- May be painful
|
| Allergic or inflammatory |
| Contact dermatitis | - Pruritic eczematous erythema with edema, vesicles, and distinct border
- Located around area of contact with allergen
- Patient may recall exposure
|
| Insect bite/sting | - Painful or pruritic erythema expanding from site of bite
- Examination may reveal bite marks
- Some patients may not recall bite
|
| Fixed drug reaction | - Round or oval erythematous or violaceous patch or patches
- Located on any part of body
- Begins 30 minutes to 8 hours after drug administration
|
| Reaction to foreign body implant (eg, metal, mesh, silicone) | - Erythema overlying site of known foreign material
|
| Injection site reaction | - Erythema expanding from site of injection
|
| Eosinophilic cellulitis (Well syndrome) | - Recurrent erythematous patches or plaques
- Pruritic and non-tender
|
| Miscellaneous |
| Compartment syndrome* | - Tense, firm, "wood-like" edema
- Pain out of proportion to exam
|
| Panniculitis | - Palpable nodules or plaques on deep palpation
|
| Malignancy* | - Slowly progressive erythematous patches or plaques
- Examples include Inflammatory breast cancer and extramammary Paget disease of the genitalia or perineum
|
| Calciphylaxis | - Extremely painful indurated nodules or plaques that progress to necrotic ulcers
- Occurs in patients with chronic kidney disease, especially those on hemodialysis
|
| Radiation recall | - Painful erythema at site of prior radiation therapy
- Triggered by certain chemotherapeutic agents
- Occurs minutes to months after drug administration
|
