It is usually diagnosed clinically, and is distinguished from generalized patchy dermatitis, which has ill-defined borders. Nummular dermatitis is a condition confined to the skin. Such non-invasive colonization does not require antibiotic therapy but will be eradicated by relieving the dermatitis with topical steroids. 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.