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Rashes and Skin Lesions : LABORATORY AND DIAGNOSTIC STUDIES

Diascopy
Diascopy is used to assess for blanching on pressure and is accomplished by pressing a glass or clear plastic slide on the lesion. Diascopy is most helpful in evaluating purpuric lesions; blood that is outside vessels (as in petechiae) will not blanch, whereas blood that is entrapped within dilated vessels (as in telangiectasias) will demonstrate this phenomenon.

Dermoscopy
Dermoscopy uses a skin surface microscope (dermatoscope) with or without the application of oil on a skin lesion to illuminate and magnify a lesion. This technique allows a more detailed inspection of the surface 335 of pigmented skin lesions to confirm a diagnosis of melanoma and to determine which skin lesions require biopsy or removal. Dermoscopy requires special training and expertise.

Wood’s Light
Long-wave ultraviolet (UV) light is used in the diagnosis of lesions caused by fungal infections. Many but not all fungal rashes fluoresce different colors. Trichophyton organisms and Tinea tonsurans, dermatophytes that are frequently identified in tinea eruptions in the United States, do not fluoresce; Microsporum organisms, which can cause tinea eruptions, do fluoresce.

Skin Scraping and Potassium Hydroxide Preparation
Microscopically examine a sample of cells retrieved from a lesion, assessing for the presence of fungal or dermatophytic spores and hyphae. The lesion should be gently scraped using a scalpel (collect cells from an active area such as the border of the lesion); the cells are treated with a drop of 20% potassium hydroxide (KOH) and then warmed or allowed to stand a few minutes to soften the keratin. The addition of 40% dimethyl sulfoxide (DMSO) to the KOH solution accelerates diagnosis. Chlorazol black E stain highlights fungal hyphae as dark, blue-black against a light gray background.

Tzanck Smear
In a Tzanck smear, an indirect test for herpes virus infections (herpes simplex virus, herpes zoster), cells are retrieved by swabbing the base of a lesion (usually a vesicle), smearing it onto a glass slide, and then staining it with Giemsa or Wright solution. Examined microscopically, the presence of multinucleated giant cells confirms the presence of herpes virus, but cannot differentiate between herpes simplex virus or varicella-zoster virus infections. Viral culture is diagnostic.

Bacterial or Viral Culture
For a bacterial culture, exudate from a lesion is collected on a sterile swab and cultured for growth. Gram staining may also be done. When a bacterial isolate is known, antibiotic sensitivity testing is performed. For a viral culture, cells from the base of a lesion
(usually a vesicle) are collected on a Dacron swab and cultured for identification of viral infections, particularly HSV or HZ.

Punch Biopsy
In a punch biopsy, a cylindrical-shaped tissue sample is assessed histopathologically for identification. Select a punch size about 3 to 4 mm larger than the lesion or sample an active area if the lesion is large. First the skin is cleansed and local anesthesia is administered. While stretching the skin with the other hand, gently rotate the biopsy instrument while exerting slight downward pressure. When well into the dermis, remove the punch and excise the sample at its base. The defect may be closed using electrocautery, with suture(s), or left open to heal by second intention. Place the fresh specimen on gauze with normal saline for immediate transport to pathology to be processed. If the specimen is being sent for culture or immunofluorescent staining, place in a preservative such as formaldehyde solution.

Excisional Biopsy
In excisional biopsy, a tissue sample is assessed histopathologically for identification. Excise the entire lesion, usually making an elliptical incision around the lesion beyond its margins. Excise the base and close the defect with sutures or cauterize bleeding vessels. Handle the specimen in the same manner used for a punch biopsy.