Lesions of urticaria, whether acute or chronic, resolve within few hours, leaving behind no discoloration or other traces. Lesions are usually pruritic.

In contradistinction, lesions of UV, usually last longer than 24 h and up to a few days. Upon resolution, discoloration is almost invariable and may appear slightly purplish, purpuric, or brownish. Lesions of UV produce a burning rather than itchy sensation.

The prodromal pre-blistering eruption of pemphigoid is characteristically urticarial. Unlike lesions of urticaria, the non-bullous lesions of pemphigoid are more infiltrated and persist for days to weeks, if not treated. Not infrequently, very close inspection may reveal one or more minute, clear vesicles within the urticarial plaque, including at the border. The distribution of lesions of urticarial pemphigoid may not respect the usual distribution of lesions in patients with bullous pemphigoid, which favors skin folds.

Lesions of Sweet syndrome are often fiery red, burning and painful, may be tender, often boggy, and persistent. Their classic acral distribution is a good clue to the diagnosis. In the full syndrome, also referred to as classic Sweet syndrome, fever and leukocytosis are major clues.

An eruption of red edematous plaques may rarely be caused by neutrophilic eccrine hidradenitis. Many patients would be known to have an associated systemic illness or be on systemic chemotherapy. Lesions may favor the skin of the palms and soles.

EM is almost invariably symmetrical and the majority of patients have at least some target lesions. FDE lesions tend to be few, larger than lesions of EM and scattered asymmetrically. They often favor mucocutaneous junctions of the mouth and genitalia, and are often pigmented.