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Laboratory data are non-contributory (the diagnosis is made on a clinical basis).
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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.
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The differential diagnosis also includes conditions presenting with apparently targetoid lesions such as erythema multiforme, typical urticaria and urticaria multiformis.
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Like in other vasculitides, the platelet count is often slightly increased.
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Complement levels are normal, antinuclear or antineutrophil cytoplasmic auto-antibodies are negative.
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Urinalysis is usually normal (except in cases triggered by a urinary tract infection).
