Return To The Previous Page
Buy a Package
Number Of Visible Items Remaining : 3 Item

Office-based dermatologic diagnostic procedures

Office-based dermatologic diagnostic procedures
Beth G Goldstein, MD
Adam O Goldstein, MD, MPH
Section Editor:
Robert P Dellavalle, MD, PhD, MSPH
Deputy Editor:
Rosamaria Corona, MD, DSc
Literature review current through: Feb 2023. | This topic last updated: Apr 14, 2021.

INTRODUCTION — Several dermatologic diagnostic procedures (eg, Wood's lamp examination, potassium hydroxide [KOH] preparation, Tzanck smear) can be performed at the bedside, in the clinic, or in the clinician's office to confirm or exclude a suspected diagnosis or to differentiate among diagnoses [1,2]. Although additional tests, such as histopathology, cultures, or polymerase chain reaction, are usually performed for a more precise diagnosis, the simple procedures reviewed in this topic represent valuable diagnostic tools for several infectious and noninfectious diseases, especially in limited-resource settings. These include:

Gram stain

KOH preparation

Fungal culture

Scabies preparation

Tzanck smear

Wood's lamp examination

Dermoscopy, skin biopsy, and nail biopsy are discussed separately.

(See "Overview of dermoscopy".)

(See "Dermoscopic evaluation of skin lesions".)

(See "Skin biopsy techniques".)

(See "Nail biopsy: Indications and techniques".)

GRAM STAIN — Gram stain is widely used for the rapid identification of bacteria in skin lesions and for guiding empiric antibiotic treatment for gram-positive and gram-negative bacterial infections. The technique and interpretation of Gram staining are described in detail elsewhere. (See "Approach to Gram stain and culture results in the microbiology laboratory".)

POTASSIUM HYDROXIDE PREPARATION — The potassium hydroxide (KOH) preparation is the simplest method to microscopically identify fungi or yeasts from epidermal skin scrapings, hair roots, or nail clippings. KOH dissolves epidermal keratinocytes, allowing for easier demonstration and identification of organisms. KOH preparation is indicated to identify dermatophyte (eg, tinea pedis, manus, corporis, cruris, capitis, onychomycosis) and yeast infections (eg, pityriasis versicolor, candidiasis). KOH examination may yield false-negative results in 12 to 24 percent of cases [3-5].


Alcohol preparations.

No. 15 blade or glass slide for scraping.

Glass slide and coverslip.

KOH 10% to 20% solution with or without dimethylsulfoxide (DMSO); heating is not needed if DMSO is used.

Alcohol lamp.


Hemostats or 2x2 gauze for scalp lesions, tongue blade for oral lesions, and nail clippers or curette for subungual debris for nail lesions.


Clean the skin of any lotions or creams with an alcohol preparation.

Obtain a specimen. For the skin, use either a no. 15 blade or the side of a glass slide to scrape material loose from a leading edge; if there are pustules or vesicles, scrape the roof onto a glass slide. For oral candidiasis, use a tongue blade to scrape white plaques onto a slide. For nail infections, remove as much nail as possible with nail clippers to expose the most proximally involved area, then scrape the subungual debris onto a slide with a 1 to 2 mm curette or a no. 15 blade. For hair infections, pluck 5 to 10 hairs from an active scaling area with a hemostat and place them on a slide or rub a 2x2 gauze vigorously on an area of alopecia and scaling, placing the broken-off hairs onto a glass slide.

Apply two or three drops of KOH to the slide, then apply the coverslip.

If KOH without DMSO is used, heat the slide gently over an alcohol lamp or with a lighter for two or three seconds until just before it starts to boil. This will hasten the breakdown of cell wall components and make it easier to see fungal remnants.

Examine at 10 times magnification, using the lowest light possible (lowering the condenser is helpful). Examine at 40 times magnification, if necessary, to confirm the presence of hyphae in dermatophyte infections (picture 1A) and pseudohyphae or yeast forms for Candida or Pityrosporum infections (picture 1B). Use the fine focus to demonstrate refractile properties of organisms compared with epidermal cell walls.

For nail specimens, if the KOH with DMSO does not show the organism, you can allow the preparation to sit for 20 to 30 minutes, then re-examine. This helps the thick keratin dissolve and aids in examination of fungal elements. The identification of fungal elements may be enhanced by adding to the KOH preparation a drop of black (chlorazol black) or blue (Swartz-Lamkins) stain [6]. (See "Onychomycosis: Epidemiology, clinical features, and diagnosis", section on 'Diagnostic tests'.)

FUNGAL CULTURE — Fungal cultures are especially useful in hair or nail infections to identify the fungal species or to differentiate dermatophytes from yeast or mold infections. This differentiation can influence the choice of therapy. In contrast, fungal cultures are rarely needed to diagnose dermatophyte or yeast infections of the skin. Fungal cultures may yield false-negative results in over 40 percent of cases [3-5].


No. 15 blade or glass slide for scraping

Hemostat for plucking hairs, if needed

Dermatophyte test medium for cultures

Sterile urine cup or similar, to send specimen to laboratory (laboratory may be specific in requirements)


Obtain a specimen. For hair, remove 5 to 10 hairs from an involved, scaling area with one quick motion using a hemostat or rub a 2x2 gauze vigorously on an area of alopecia and scaling, placing the broken-off hairs into the media. After placing the hairs in the culture medium, loosely recap the bottle and store it in a dark cabinet. For nails, remove a portion of the involved nail with nail clippers to expose the most proximal involved area. Using a no. 15 blade or a curette, scrape the proximal subungual debris into the culture medium. Nail clippers can be used to remove the involved proximal areas but cover the nail with 4x4 gauze to avoid having a specimen fly across the room.

After embedding some of the specimen in the medium, apply the cap loosely and store at room temperature in a dark cabinet.

Examine the specimen after 5 to 10 days and up to two weeks, looking for the phenol indicator in the test medium to turn from yellow to red.

Monomorphous colony growth indicates a dermatophyte infection. Yeast looks like creamy, discrete colonies (picture 2).

Bacterial contaminants and nonpathogenic molds also can grow; it may be necessary to send the specimen to a mycology laboratory if the diagnosis is in doubt.

SCABIES PREPARATION — The scabies preparation is described in detail separately. (See "Scabies: Epidemiology, clinical features, and diagnosis", section on 'Scabies preparation'.)

TZANCK SMEAR — The Tzanck smear, first introduced in 1947, is a simple, rapid, and inexpensive test that can be easily performed in the clinic or clinician's office. Although it is typically employed for a rapid diagnosis of herpes simplex virus (HSV) infections, it can also be used as an initial diagnostic test for other viral and bacterial infections and for several noninfectious inflammatory conditions [1,7-10]. A number of stains for Tzanck smear are commercially available.


Alcohol preparation

No. 15 blade

Cotton swab

Glass slide and coverslip

Mineral oil


For modified, quick Tzanck – spray-on cytology fixative and ready-to-use urine sediment stain

For routine staining – 95% methanol; distilled water; and Giemsa, Wright's, or Hansel stain


Clean an area with an intact vesicle or blister for best results. If no intact vesicles are present, use the edge of the most recently appearing erosion or ulcer.

Remove a blister roof with a no. 15 blade, blot any excess blister fluid, then scrape the base of the vesicle, erosion, or ulcer with the scalpel blade, spreading a thin layer of the resultant material onto a glass slide.

For a modified, quick Tzanck test, immediately fix the slide with spray cytology fixative and air dry for 5 to 10 minutes. Flood the slide with urine sediment stain for 30 to 60 seconds, rinse gently with tap water, and air dry. The modified test is easier, quicker, and utilizes fewer supplies than the routine test.

For routine staining, fix the specimen by flooding the slide with 95% methanol for five seconds, then air dry for one to two minutes. Flood the slide with nuclear stain (Wright's, Giemsa, or Hansel stain) for 30 to 60 seconds. Add distilled water to the slide for 30 seconds, then flood it with distilled water to remove any remaining stain. Flood the slide again with 95% methanol for Hansel stain. Air dry without blotting.

Observe the slide initially at 40 times magnification to find areas where individual cells are best identified (ie, not clumped together). Then, use oil immersion at 100 times magnification to identify multinucleated giant cells (ie, where nuclei are molded together in giant epithelial cells). Using microscope settings under high light and with the condenser up is most helpful.

Viral infections — The Tzanck smear is most commonly used for the diagnosis of herpes simplex and varicella-zoster infections; it can also be used in immunosuppressed patients to differentiate molluscum contagiosum (MC) from deep fungal infections:

Herpes simplex virus – Typical cytologic findings in herpes simplex virus (HSV) infection include multinucleated giant cells, acantholytic cells, keratinocyte ballooning, and nuclear molding (picture 3) [9]. Tzanck smear's sensitivity for the diagnosis of HSV infection varies from 40 to over 80 percent, depending upon the experience of the examiner, and is generally lower than viral cultures [1]. Tzanck smear cannot distinguish among herpes simplex virus type 1 (HSV-1), herpes simplex virus type 2 (HSV-2), and varicella zoster virus. Although the diagnosis of a specific HSV infection can be confirmed by viral culture, direct fluorescent antibody testing, and type-specific serology, real-time polymerase chain reaction (PCR) has emerged as the most sensitive method to confirm HSV infection in clinical specimens obtained from mucocutaneous sites or cerebrospinal fluid. (See "Epidemiology, clinical manifestations, and diagnosis of herpes simplex virus type 1 infection", section on 'Diagnosis'.)

Molluscum contagiosum – The Tzanck smear can be used to confirm the diagnosis of molluscum contagiosum (MC) when the clinical presentation is atypical or, in immunosuppressed patients, to differentiate MC from deep fungal infections (eg, cryptococcosis, histoplasmosis, coccidioidomycosis) that present with umbilicated papules mimicking MC [11].

For the Tzanck smear, the central part of the lesion is scraped with a no. 15 scalpel blade and applied in a thin layer on a glass slide. The characteristic molluscum bodies present as large, ovoid, eosinophilic, cytoplasmic inclusions and may be easily identified with hematoxylin and eosin stain. (See "Molluscum contagiosum", section on 'Diagnosis'.)

Bacterial infections — In bacterial infections, such as bullous impetigo, cytologic findings include dyskeratotic acantholytic cells, abundant neutrophils, and clumps of cocci.

Leishmaniasis — Rapid bedside diagnosis of cutaneous leishmaniasis (picture 4) can be performed using a Tzanck preparation obtained by scraping the lesion using a scalpel and pushing in one direction until blood oozes from the inflamed border of the lesion; the blood drops are then applied to a slide as a thin smear or "thick drop" that is let dry at room temperature [12]. The amastigote protozoa can be easily identified by Wright-Giemsa stains as blue, "swarm of bees," round or oval bodies (Leishman-Donovan bodies) in the cytoplasm of macrophage mononuclear cells (picture 5). In a study including 72 patients with cutaneous leishmaniasis who underwent lesion scraping, biopsy, and PCR for diagnostic confirmation, the thick drop Tzanck was more sensitive than biopsy and thin smear for the diagnosis of leishmaniasis (64, 44, and 39 percent, respectively) [12]. (See "Cutaneous leishmaniasis: Clinical manifestations and diagnosis".)

Noninfectious diseases — Tzanck smear is the test of choice for the diagnosis of the benign pustuloses of the newborn [13]. Moreover, it can be used as an initial screening test for patients with suspected autoimmune blistering diseases [2,9]:

In neonates with erythema toxicum neonatorum (picture 6A-B), a common pustular eruption affecting neonates in the first few days of life, a Tzanck smear will a reveal the presence of numerous eosinophils and absent bacteria. In acropustulosis of infancy and transient pustular melanosis, Tzanck smear demonstrates abundant neutrophils, few eosinophils, and absent bacteria. (See "Vesicular, pustular, and bullous lesions in the newborn and infant", section on 'Benign vesiculopustular eruptions'.)

On a Tzanck smear, lesions of pemphigus vulgaris (picture 7) typically show numerous round, acantholytic cells with enlarged nuclei and basophilic cytoplasm with a more intensely stained peripheral rim [9]. In contrast, in lesions of bullous pemphigoid, acantholytic cells are absent and there are abundant eosinophils. The definitive diagnosis of pemphigus vulgaris and bullous pemphigoid requires a skin biopsy for histopathology and direct immunofluorescence and the demonstration of circulating autoantibodies by enzyme-linked immunosorbent assay. (See "Pathogenesis, clinical manifestations, and diagnosis of pemphigus", section on 'Diagnosis' and "Clinical features and diagnosis of bullous pemphigoid and mucous membrane pemphigoid", section on 'Diagnosis'.)

WOOD'S LAMP EXAMINATION (BLACK LIGHT) — The Wood's lamp is a device that emits ultraviolet (UV) light using a filter selective for wavelengths in the 320 to 400 nm region of the UV spectrum, with a peak at 365 nm [14]. Under Wood's light, normal skin has a blue fluorescent appearance, whereas different skin diseases have characteristic fluorescent patterns. These include [15,16]:

Pigmentary disorders

Vitiligo – Under Wood's light, vitiligo lesions (picture 8) appear bright white and sharply delineated, as a result of the autofluorescence of dermal collagen. (See "Vitiligo: Pathogenesis, clinical features, and diagnosis".)

Melasma – Wood's lamp examination may assist in identifying the location of pigment in melasma, especially in individuals with lighter complexions. Epidermal melanosis often manifests as well-circumscribed pigmentation with accentuated borders. In contrast, dermal melanosis typically appears poorly circumscribed and is not accentuated under Wood's lamp illumination. (See "Melasma: Epidemiology, pathogenesis, clinical presentation, and diagnosis".)

Progressive macular hypomelanosis – The hypopigmented lesions of progressive macular hypomelanosis (picture 9) demonstrate a characteristic punctiform, orange-red, follicular fluorescence when examined under Wood's light. (See "Acquired hypopigmentation disorders other than vitiligo", section on 'Progressive macular hypomelanosis'.)


Dermatophyte infections – Only some of the dermatophytes that cause tinea capitis (picture 10A-B) produce fluorescence. These include members of the genus Microsporum (eg, Microsporum audouinii and Microsporum canis). Infected hairs produce a blue-green fluorescence under the Wood's light. Members of the genus Trichophyton, such as Trichophyton schoenleinii, produce only a faint, dull blue fluorescence. (See "Tinea capitis".)

Erythrasma – In erythrasma, a superficial bacterial infection caused by Corynebacterium minutissimum most often involving the intertriginous areas (picture 11A-B), examination with the Wood's lamp reveals a coral-red fluorescence due to bacterial coproporphyrin III (picture 12). (See "Erythrasma".)

Pityriasis versicolor – In pityriasis versicolor, a common, superficial fungal skin infection caused by yeasts in the genus Malassezia (picture 13A-B), the yeasts emit a yellow-orange fluorescence. Pseudomonas reveals a yellow-green fluorescence. (See "Tinea versicolor (pityriasis versicolor)".)

Lentigo maligna – The use of the Wood's lamp may improve the detection of the borders of lentigo maligna (picture 14) before surgical excision by accentuating the hyperpigmentation in the epidermis. (See "Lentigo maligna: Clinical manifestations, diagnosis, and management".)

Porphyria cutanea tarda – A presumptive diagnosis of porphyria cutanea tarda (picture 15), a metabolic disorder caused by the altered activity of enzymes in the heme biosynthetic pathway, is possible if pink or orange-red fluorescence is seen in urine examined under a Wood's light. (See "Porphyria cutanea tarda and hepatoerythropoietic porphyria: Pathogenesis, clinical manifestations, and diagnosis".)


Wood's lamp or hand-held black light source

Electrical source

Darkened room


The examination with a Wood's lamp should be performed in a dark room, with the light source at 4 to 5 inches from the skin surface.

If the patient has recently bathed, fluorescence may be reduced.

Perfumes, makeup, skin care products, topical medications, fabric fibers, and scale may also fluoresce and produce a false-positive result.


Several dermatologic diagnostic procedures (eg, potassium hydroxide [KOH] preparation, Tzanck smear, Wood's lamp examination) can be performed at the bedside, in the clinic, or in the clinician's office to confirm or exclude a suspected diagnosis or to differentiate among diagnoses. (See 'Introduction' above.)

Dermoscopic examination of pigmented and nonpigmented skin lesions is discussed separately. (See "Overview of dermoscopy" and "Dermoscopic evaluation of skin lesions".)

The technique and interpretation of Gram staining for the rapid identification of bacteria in skin lesions are described in detail elsewhere. (See "Approach to Gram stain and culture results in the microbiology laboratory".)

The KOH preparation is the simplest method to microscopically identify fungi or yeasts from epidermal skin scrapings, hair roots, or nail clippings (picture 1A-C). (See 'Potassium hydroxide preparation' above.)

Fungal cultures (picture 2) are especially useful in hair or nail infections to identify the fungal species or to differentiate dermatophytes from yeast or mold infections. However, the sensitivity of culture may not be much better than KOH examination, and results are not available for four to six weeks. (See 'Fungal culture' above and "Onychomycosis: Epidemiology, clinical features, and diagnosis", section on 'Diagnosis' and "Tinea capitis", section on 'Culture'.)

The Tzanck smear is a simple, rapid, and inexpensive test typically employed for a rapid diagnosis of herpes virus infections. However, it can also be used as a diagnostic test for a number of other infectious and noninfectious skin diseases presenting with vesiculopustular or bullous lesions. (See 'Tzanck smear' above.)

The Wood's lamp is a device that emits ultraviolet (UV) light in the 320 to 400 nm region of the UV spectrum. Examination under the Wood's light may assist in the diagnosis of several skin diseases, including vitiligo, melasma, tinea capitis, erythrasma, and pityriasis versicolor. (See 'Wood's lamp examination (black light)' above.)

The scabies preparation is described in detail separately. (See "Scabies: Epidemiology, clinical features, and diagnosis", section on 'Scabies preparation'.)

Skin biopsy techniques are discussed in detail separately. (See "Skin biopsy techniques".)

  1. Wanat KA, Dominguez AR, Carter Z, et al. Bedside diagnostics in dermatology: Viral, bacterial, and fungal infections. J Am Acad Dermatol 2017; 77:197.
  2. Micheletti RG, Dominguez AR, Wanat KA. Bedside diagnostics in dermatology: Parasitic and noninfectious diseases. J Am Acad Dermatol 2017; 77:221.
  3. Karimzadegan-Nia M, Mir-Amin-Mohammadi A, Bouzari N, Firooz A. Comparison of direct smear, culture and histology for the diagnosis of onychomycosis. Australas J Dermatol 2007; 48:18.
  4. Lilly KK, Koshnick RL, Grill JP, et al. Cost-effectiveness of diagnostic tests for toenail onychomycosis: a repeated-measure, single-blinded, cross-sectional evaluation of 7 diagnostic tests. J Am Acad Dermatol 2006; 55:620.
  5. Weinberg JM, Koestenblatt EK, Tutrone WD, et al. Comparison of diagnostic methods in the evaluation of onychomycosis. J Am Acad Dermatol 2003; 49:193.
  6. Shi VY, Lio PA. In-office diagnosis of cutaneous mycosis: a comparison of potassium hydroxide, Swartz-Lamkins, and chlorazol black E fungal stains. Cutis 2013; 92:E8.
  7. Durdu M, Baba M, Seçkin D. More experiences with the Tzanck smear test: cytologic findings in cutaneous granulomatous disorders. J Am Acad Dermatol 2009; 61:441.
  8. Durdu M, Baba M, Seçkin D. The value of Tzanck smear test in diagnosis of erosive, vesicular, bullous, and pustular skin lesions. J Am Acad Dermatol 2008; 59:958.
  9. Kelly B, Shimoni T. Reintroducing the Tzanck smear. Am J Clin Dermatol 2009; 10:141.
  10. Panwar H, Joshi D, Goel G, et al. Diagnostic Utility and Pitfalls of Tzanck Smear Cytology in Diagnosis of Various Cutaneous Lesions. J Cytol 2017; 34:179.
  11. Fridlington E, Colome-Grimmer M, Kelly E, Kelly BC. Tzanck smear as a rapid diagnostic tool for disseminated cryptococcal infection. Arch Dermatol 2006; 142:25.
  12. Saab M, El Hage H, Charafeddine K, et al. Diagnosis of cutaneous leishmaniasis: why punch when you can scrape? Am J Trop Med Hyg 2015; 92:518.
  13. Van Praag MC, Van Rooij RW, Folkers E, et al. Diagnosis and treatment of pustular disorders in the neonate. Pediatr Dermatol 1997; 14:131.
  14. Sharma S, Sharma A. Robert Williams Wood: pioneer of invisible light. Photodermatol Photoimmunol Photomed 2016; 32:60.
  15. Klatte JL, van der Beek N, Kemperman PM. 100 years of Wood's lamp revised. J Eur Acad Dermatol Venereol 2015; 29:842.
  16. Pflederer RT, Wuennenberg JP, Foote C, et al. Use of Wood's lamp to diagnose progressive macular hypomelanosis. J Am Acad Dermatol 2017; 77:e99.
Topic 5562 Version 15.0

Do you want to add Medilib to your home screen?