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
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. |Favorite Table|Download (.pdf) Table 4-1. Potassium hydroxide (KOH) examination for superficial fungal infections.
#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 MycoselTM or MycobioticTM, 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 ...