Spreading dermal erythema, tenderness and frank pus are indicators of invasive infection and the need for antibiotic therapy. No racial predilection has been observed. Nummular dermatitis is more common in males. The incidence peaks in the sixth and seventh decades. Another common population is atopics in the second and third decades.
Nummular dermatitis is rare in children.
Plaques may develop central clearing that resembles tinea corporis, which tends to be less vesicular, has a narrower border, and is duller in color. Tinea should be excluded by scraping and microscopic analysis of a KOH preparation. Contact dermatitis may have a pattern that approximates the way the offending agent came in contact with the skin, such as a linear pattern. It may become chronic in the setting of repeated exposure such as with chromate and formaldehyde. The patient may recall contact to an allergen such as poison ivy.