Nummular Dermatitis (Nummular Eczema) Workup

Updated: Nov 20, 2020
  • Author: Jami L Miller, MD; Chief Editor: William D James, MD  more...
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Laboratory Studies

Tinea corporis should be excluded by scraping and microscopically analyzing a potassium hydroxide preparation of a lesion.

For lesions that have erythema spreading away from the lesions, suggesting cellulitis, swab culture of the exudate may be helpful. First-generation cephalosporins are still usually effective first-line treatment.  As methicillin-resistant Staphylococcus aureus becomes more common in a community, the culture results help choose appropriate antibiotics in treatment-resistant cases of documented secondarily infected lesions.

Consider a workup for infection, such as with H pylori and for giardiasis, if appropriate history is suggestive of infection. [22]



A skin biopsy may be performed. The findings are nonspecific, but they may help differentiate nummular dermatitis from tinea corporis, psoriasis, a fixed drug eruption, or cutaneous T-cell lymphoma.

Some studies have recommended patch testing in patients with refractory nummular dermatitis. One study found that 50% of 56 patients with nummular eczema showed positive reactions on patch testing, and other research identified positive patch testing in 23 of 50 patients with nummular dermatitis. [29, 30]  A 2012 study strongly recommended patch testing in nummular dermatitis, as it showed that 332 (32.5%) of 1022 patients with nummular dermatitis had positive patch test results for one or more allergens. [5]


Histologic Findings

Biopsy findings mirror the evolution of the lesion. In the early stages, a nonspecific infiltrate is present with spongiosis, vesicles, and a predominant lymphocytic infiltrate. Eosinophils may be observed in the papillary dermis. Chronic lesions demonstrate epidermal hyperplasia, hyperkeratosis, and a pronounced granular cell layer. The papillary dermis may be fibrotic, with a perivenular infiltrate of lymphocytes and monocytes.

Lymphocytes are predominately CD8+ in the epidermis and CD4+ in the dermis. Mast cell–derived interleukin 4 appears to be involved in activation of the T lymphocytes.