Laboratory data are non-contributory (the diagnosis is made on a clinical basis).
Because of the violent and often apparently life-threatening presentation, the laboratory work-up aims at excluding diseases such as acute systemic infections. The further differential diagnosis includes drug-induced skin lesions, discolored leg syndrome after vaccination, and nonaccidental traumatic skin lesions.
The differential diagnosis also includes conditions presenting with apparently targetoid lesions such as erythema multiforme, typical urticaria and urticaria multiformis.
Like in other vasculitides, the platelet count is often slightly increased.
Complement levels are normal, antinuclear or antineutrophil cytoplasmic auto-antibodies are negative.
Urinalysis is usually normal (except in cases triggered by a urinary tract infection).
