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Diagnostic Tests for Skin Disorders


Julia Benedetti

, MD, Harvard Medical School

Reviewed/Revised Dec 2021 | Modified Sep 2022
Topic Resources

Diagnostic tests are indicated when the cause of a skin lesion or disease is not obvious from history and physical examination alone. These include


There are several types of skin biopsy:

  • Punch

  • Shave

  • Wedge excision

In a punch biopsy, a tubular punch (diameter usually 4 mm) is inserted into deep dermal or subcutaneous tissue to obtain a specimen, which is snipped off at its base.

Shaving with a scalpel or razor blade may be done for more superficial lesions. Bleeding is controlled by aluminum chloride solution or electrodesiccation; large incisions are closed by sutures.

Wedge excision of skin using a scalpel can be done for larger or deeper biopsies.

Pigmented lesions are often excised for histologic evaluation of depth; if too superficial, definitive diagnosis may be impossible. Diagnosis and cure can often be achieved simultaneously for most small tumors by complete excision that includes a small border of normal skin.


Wood light

A Wood light (black light) can help clinicians diagnose and define the extent of lesions (eg, borders of pigmented lesions before excision). It can help distinguish hypopigmentation from depigmentation (depigmentation of vitiligo Vitiligo Vitiligo is a loss of skin melanocytes that causes areas of skin depigmentation of varying sizes. Cause is unknown, but genetic and autoimmune factors are likely. Diagnosis is usually clear... read more Vitiligo fluoresces ivory-white and hypopigmented lesions do not). Erythrasma Erythrasma Erythrasma is an intertriginous infection with Corynebacterium minutissimum that is most common among patients with diabetes and among people living in warmer climates. Diagnosis is clinical... read more Erythrasma fluoresces a characteristic bright orange-red. Tinea capitis Tinea Capitis (Scalp Ringworm) Tinea capitis is a dermatophyte infection of the scalp. Diagnosis is by clinical appearance and by examination of plucked hairs or hairs and scale on potassium hydroxide wet mount. Treatment... read more Tinea Capitis (Scalp Ringworm) caused by Microsporum canis and M. 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 Herpes Simplex Virus (HSV) Infections Herpes simplex viruses (human herpesviruses types 1 and 2) commonly cause recurrent infection affecting the skin, mouth, lips, eyes, and genitals. Common severe infections include encephalitis... read more Herpes Simplex Virus (HSV) Infections and herpes zoster Herpes Zoster Herpes zoster is infection that results when varicella-zoster virus reactivates from its latent state in a posterior dorsal root ganglion. Symptoms usually begin with pain along the affected... read more Herpes Zoster , and is done when active intact vesicles are present. Tzanck testing cannot distinguish between herpes simplex and herpes zoster infections. An intact blister is the preferred lesion for examination. The blister roof is removed with a sharp blade, and the base of the unroofed vesicle is scraped with a #15 scalpel blade. The scrapings are transferred to a slide and stained with Wright stain or Giemsa stain. Multinucleated giant cells are a sign of herpes infection.


Diascopy is used to determine whether erythema in a lesion is due to blood within superficial vessels (inflammatory or vascular lesions) or is due to hemorrhage (petechiae or purpura). A microscope slide is pressed against a lesion (diascopy) to see whether it blanches. Hemorrhagic lesions do not blanch; inflammatory and vascular lesions do. Diascopy can also help identify sarcoid skin lesions, which, when tested, turn an apple jelly color.

Drugs Mentioned In This Article

Drug Name Select Trade
Drysol, Hypercare, Xerac AC
Fleet, Kondremul, Liqui-Doss, Muri-Lube
NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
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