The clinical differential diagnosis includes pyoderma gangrenosum, infection, erythema multiforme, adverse drug reactions, and urticaria. Recurrences are common and affect up to one third of patients. Laboratory studies Studies show a moderate<unk>  (less than 50%), elevated ESR (greater than 30 mm/h) (90%), and a slight increase in alkaline phosphatase (83%). Skin biopsy shows a papillary and mid-dermal mixed infiltrate of polymorphonuclear leukocytes with nuclear fragmentation and histiocytic cells. The infiltrate is predominantly perivascular with endothelial-cell swelling in some vessels, but vasculitic changes (blood clots; deposition of fibrin, complement, or immunoglobulins within the vessel walls; red blood cell extravasation;inflammatory infiltration of vascular walls) are absent in early lesions.Perivasculitis occurs secondarily, because of cytokines released by the lesional neutrophils. True transmural vasculitis is not an expected finding histopathologically in SS. Definition Sweet described a disease with four features: fever; leukocytosis; acute, tender, red plaques; and a papillary dermal infiltrate of neutrophils. This led to the name acute<unk> . Larger series of patients showed that fever and<unk>  are not consistently present. The diagnosis is based on the two constant features, a typical eruption and the characteristic histologic features; thus the eponym "Sweet's syndrome" is used.
Febrile neutrophilic dermatosis
Behçet's disease
Streptococcal intertrigo
Drug eruption
Poikiloderma vasculare atrophicans
Toxic epidermal necrolysis
Restrictive dermopathy
Cutaneous lupus mucinosis