Diagnostic Procedures

INTRODUCTION TO CHAPTER

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A few simple diagnostic skin procedures such as potassium hydroxide (KOH) examinations, Tzanck smears, and scrapings for scabies can be valuable tests to confirm a clinical diagnosis. However, these tests can have false-positive and false-negative results which are typically due to the following problems:

Improper site or lesion selection
Faulty collection technique
Failure to systematically scan entire specimen
Artifacts in the specimen

Polymerase chain reaction (PCR) tests are replacing some of the diagnostic tests used in dermatology, but they are more expensive and are not yet widely available in all clinical settings.^1–3

KOH EXAMINATIONS AND FUNGAL CULTURES

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KOH examinations (Table 4-1) are a cost-effective method for the detection of superficial fungal skin infections. In the hands of an experienced clinician this test has a high level of specificity and sensitivity. However, cotton or nylon fibers from clothing and socks can mimic fungal hyphae, a mosaic artifact created by cell walls can also mimic fungal hyphae, and air bubbles can mimic spores. Some of these false positives can be reduced by the use of special stains such as Chicago Sky Blue⁴ or Chlorazol Black E. Fungal cultures are another option for detection of fungal infections. Dermatophyte Test Media (DTM), a modified Sabouraud agar contains an indicator dye that turns red within 7 to 14 days in the presence of viable dermatophytes (Figure 4-3).

Table 4-1.Potassium hydroxide (KOH) examination for superficial fungal infections.

Equipment needed

#15-scalpel blade (for skin and nails), small curette (for nails), tweezers or needle holder, and swab or gauze (for scalp and hair)
Microscope, glass slides, coverslip, and 20% KOH solution (plain or with dimethylsulfoxide [DMSO] or dyes such as Chlorazol Black E or Chicago Sky Blue)
If specimens are submitted for culture, a sterile urine container or Petri dish is needed for transport of the specimens to the laboratory. Modified Sabouraud agar, such as Dermatophyte Test Medium (DTM) or Mycosel^TM or Mycobiotic^TM, should be available if specimens will be directly placed onto fungal media

Techniques for specimen collection

Skin: Select an area of scale from the edge of the lesion. Clean the area with an alcohol pad. Gently scrape off the scale using a #15-scalpel blade onto a glass slide or sterile urine container. If the lesions are vesicular (eg, vesicular tinea pedis), trim off the roof of a vesicle with iris scissors and submit that as a specimen
Nails: Scrape out subungual debris using a small metal skin or ear curette or a #15-scalpel blade
Scalp: Pluck involved hairs with a needle holder or tweezers; cut the hair saving the proximal 1 to 2 cm of the hair fiber and bulb. Scalp scales can be collected for culture using a bacterial culturette swab that has been premoistened by the transport media or sterile gauze or a sterile tooth brush. The involved areas of the scalp should be vigorously rubbed with these devices

Examination of specimens

Place the skin scales or proximal hair fibers on a glass slide and cover the specimen with 2 to 4 drops of 20% KOH solution (plain or with DMSO or Chlorazol Black E or Chicago Sky Blue) to partially dissolve keratin. Apply a coverslip
Let the slide sit for 20 min
Lower the condenser on the microscope and decrease the intensity of the light
Scan the entire preparation at low power (10 ×) magnification looking for branching septate hyphae (Figure 4-1) or pseudohyphae and spores (Figure 4-2). These may be slightly refractile. Switch to high power (40 × magnification) to confirm findings
Be aware of the many causes of false-positive examinations, such as clothing fibers, hair fibers, and the cell walls of keratinocytes, which can look like hyphae. Also, air bubbles and oil droplets which can look like spores

Figure 4-1.

Potassium hydroxide (KOH) preparation. Fungal hypha with branches.

Figure 4-2.

Chicago Sky Blue and potassium hydroxide (KOH) preparation. Pseudohyphae and spores in tinea versicolor infection.

Figure 4-3.

Dermatophyte Test Media (DTM). Change in color of medium from yellow to red indicating the presence of a dermatophyte fungus.

Indications for a KOH examination or fungal culture include the presence of scaly annular plaques, areas of scale and/or alopecia on the scalp (primarily in preadolescents), vesicular lesions on the feet, and thickened nail plates.

A video demonstrating specimen collection techniques and preparation of the specimen for KOH examination can be found at www.LangeClinicalDermatology.com.

TZANCK TEST

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The Tzanck test (Table 4-2) is a cost-effective method for the detection of cutaneous herpes simplex and herpes zoster infections;¹ however, there can be significant rates of false positives and false negatives. Therefore, it is being replaced in many clinics and hospitals by PCR tests that have higher rates of sensitivity and specificity.

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