Chapter 052. Approach to the Patient with a Skin Disorder (Part 8) Tzanck Smear A Tzanck smear is a cytologic technique most often used in the diagnosis of herpesvirus infections [herpes simplex virus (HSV) or varicella zoster virus (VZV)] (see Figs. 173-1 and 173-3). An early vesicle, not a pustule or crusted lesion, is unroofed, and the base of the lesion is scraped gently with a scalpel blade. The material is placed on a glass slide, air-dried, and stained with Giemsa or Wright's stain. Multinucleated epithelial giant cells suggest the presence of HSV or VZV; culture or immunofluorescence testing must be performed to identify the specific virus. Diascopy Diascopy is designed to assess whether a skin lesion will blanch with pressure as, for example, in determining whether a red lesion is hemorrhagic or simply blood-filled. Urticaria (Fig. 52-11) will blanch with pressure, whereas a purpuric lesion caused by necrotizing vasculitis (Fig. 52-4) will not. Diascopy is performed by pressing a microscope slide or magnifying lens against a lesion and noting the amount of blanching that occurs. Granulomas often have an opaque to transparent, brown-pink "apple jelly" appearance on diascopy. Figure 52-11 Urticaria. Discrete and confluent, edematous, erythematous papules and plaques are characteristic of this whealing eruption. Wood's Light A Wood's lamp generates 360-nm ultraviolet (or "black") light that can be used to aid the evaluation of certain skin disorders. For example, a Wood's lamp will cause erythrasma (a superficial, intertriginous infection caused by Corynebacterium minutissimum) to show a characteristic coral pink color, and wounds colonized by Pseudomonas to appear pale blue. Tinea capitis caused by certain dermatophytes such as Microsporum canis or M. audouini exhibits a yellow fluorescence. Pigmented lesions of the epidermis such as freckles are accentuated, while dermal pigment such as postinflammatory hyperpigmentation fades under a Wood's light. Vitiligo (Fig. 52-12) appears totally white under a Wood's lamp, and previously unsuspected areas of involvement often become apparent. A Wood's lamp may also aid in the demonstration of tinea versicolor and in recognition of ash leaf spots in patients with tuberous sclerosis. Figure 52-12 Vitiligo. Characteristic lesions display an acral distribution and striking depigmentation as a result of loss of melanocytes. Patch Tests Patch testing is designed to document sensitivity to a specific antigen. In this procedure, a battery of suspected allergens is applied to the patient's back under occlusive dressings and allowed to remain in contact with the skin for 48 h. The dressings are removed, and the area is examined for evidence of delayed hypersensitivity reactions (e.g., erythema, edema, or papulovesicles). This test is best performed by physicians with special expertise in patch testing and is often helpful in the evaluation of patients with chronic dermatitis. FURTHER READINGS Dermatology Lexicon Project: www.futurehealth.rochester.edu/dlp2/ James WD et al: Andrews' Diseases of the Skin: Clinical Dermatology, 10th ed. Philadelphia, Elsevier, 2006 Wolff K et al (eds): Fitzpatrick's Dermatology in General Medicine, 7th ed. New York, McGraw-Hill, 2008 . Chapter 052. Approach to the Patient with a Skin Disorder (Part 8) Tzanck Smear A Tzanck smear is a cytologic technique most often used in the diagnosis of herpesvirus. suspected allergens is applied to the patient& apos;s back under occlusive dressings and allowed to remain in contact with the skin for 48 h. The dressings are removed, and the area is examined. Urticaria. Discrete and confluent, edematous, erythematous papules and plaques are characteristic of this whealing eruption. Wood's Light A Wood's lamp generates 360-nm ultraviolet