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Diagnostic tests are indicated when the cause of a skin lesion or disease is not obvious from history and physical examination alone (for patch testing, see Dermatitis: Diagnosis).
Biopsy:
A skin biopsy can be performed by a primary care physician. One procedure is a punch biopsy, in which a tubular punch (diameter usually 3 mm) is inserted into deep dermal or subcutaneous tissue to obtain a specimen, which is snipped off at its base. More superficial lesions may be biopsied by scraping with a sharp curette or shaving with a scalpel. Bleeding is controlled by aluminum chloride solution or electrodesiccation; large incisions are closed by sutures. Larger or deeper biopsies can be obtained by excising a wedge of skin with a scalpel. All pigmented lesions should be excised deeply for histologic evaluation of depth; superficial biopsies are inadequate. Diagnosis and cure are achieved simultaneously for most small tumors by complete excision that includes a small border of normal skin.
Scrapings:
Skin scrapings help diagnose fungal infections and scabies. For fungal infection, scales are taken from the border of the lesion, put onto a microscope slide, and a drop of 10 to 20% potassium hydroxide (KOH) is added. Hyphae and/or budding yeast confirm the diagnosis of tinea or candidiasis. For scabies, scrapings are taken from suspected burrows and placed directly under a coverslip with mineral oil; findings of mites, feces, or eggs confirm the diagnosis.
Wood's
light:
Wood's light (black light) can help distinguish hypopigmentation from depigmentation (depigmentation of vitiligo fluoresces ivory-white and hypopigmented lesions do not). Erythrasma fluoresces bright orange-red. Tinea capitis caused by Microsporum canis and Microsporum audouinii fluoresces a light, bright green. (Note: Most tinea capitis in the US is caused by Trichophyton species, which do not fluoresce.) The earliest clue to cutaneous Pseudomonas infection (eg, in burns) may be green fluorescence.
Tzanck
testing:
Tzanck testing can be used to diagnose viral disease, such as herpes simplex and herpes zoster, and is performed when active intact vesicles are present. Tzanck testing cannot distinguish between herpes simplex and herpes zoster infections. Cellular material is scraped with a #15 scalpel blade from the base and sides of a vesicle and stained with Wright's or Giemsa stain. Multinucleated giant cells are a sign of viral disease.
Diascopy:
Diascopy is used to distinguish between hemorrhagic and inflammatory lesions. A microscope slide is pressed against a lesion to see whether it blanches. Hemorrhagic lesions (petechiae or purpura) do not blanch; inflammatory lesions do. Diascopy is sometimes used to distinguish epidermal lesions from dermal nodules; epidermal lesions disappear, while dermal nodules remain and may turn an “apple jelly” color.
Last full review/revision November 2005
Content last modified November 2005
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