Tinea Corporis Workup

Updated: Jul 09, 2018
  • Author: Jack L Lesher, Jr, MD; Chief Editor: Dirk M Elston, MD  more...
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Workup

Laboratory Studies

A potassium hydroxide (KOH) examination of skin scrapings may be diagnostic in tinea corporis. A KOH test is a microscopic preparation used to visualize fungal elements removed from the skin's stratum corneum. The sample should be taken from the active border of a lesion because this region provides the highest yield of fungal elements. A KOH preparation from a vesicular lesion should be made from the roof of the vesicle. The KOH helps dissolve the keratin and leaves fungal elements intact, revealing numerous septate, branching hyphae amongst epithelial cells. A counterstain, such as chlorazol black E or Parker blue-black ink, may help visualize hyphae under the microscope.

A fungal culture is often used as an adjunct to KOH for diagnosis. Fungal culture is more specific than KOH for detecting a dermatophyte infection; therefore, if the clinical suspicion is high yet the KOH result is negative, a fungal culture should be obtained.

A few culture mediums are available for dermatophyte growth. Sabouraud agar containing neopeptone or polypeptone agar and glucose is often used for fungal culture. However, it does not contain antibiotics and may allow overgrowth of fungal and bacterial contaminants. Mycosel, a commonly used agar, is similar to Sabouraud agar but has antibiotics. Commonly, dermatophyte test medium (DTM) is used. It contains antibacterial (ie, gentamicin, chlortetracycline) and antifungal (ie, cycloheximide) solutions in a nutrient agar base. This combination isolates dermatophytes while suppressing other fungal and bacterial species that may contaminate the culture.

Following culture inoculation, potential fungal growth is monitored for 2 weeks.

Positive culture results vary depending on the medium used. DTM contains phenol red solution, which causes a color change from straw-yellow to bright-red under alkaline conditions, indicating a positive dermatophyte culture result. However, the color makes identification of culture morphology (particularly pigmentation) difficult. Sabouraud or Mycosel agar should be used to assess gross and microscopic colony characteristics.

If the above clinical evaluations are inconclusive, the molecular method of polymerase chain reaction for fungal DNA identification can be applied. [6]

For atypical presentations of tinea corporis, further evaluation for HIV infection and/or an immunocompromised state should be considered.

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Histologic Findings

A skin biopsy specimen with hematoxylin and eosin staining of tinea corporis demonstrates spongiosis, parakeratosis, and a superficial inflammatory infiltrate. Neutrophils may be seen in the stratum corneum, which is a significant diagnostic clue. On occasion, septate branching hyphae are seen in the stratum corneum with hematoxylin and eosin stain, but special fungal stains (eg, periodic acid-Schiff, Gomori methenamine silver) may be required.

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