Tinea Corporis Workup

Updated: Sep 17, 2020
  • Author: Shweta Shukla, 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. To perform a KOH test, the fungal lesions are scraped with a No. 10 scalpel blade. It is important to scrape the peripheral margin of the lesion, as it has the most scale. A generous amount of skin scrapings should be taken. A clean glass microscope slide should be held underneath and scale collected. A coverslip should then be placed on the scale, and a few drops of 10-20% KOH solution should be added adjacent to the coverslip and will be drawn in with capillary action. A gentle flame, such as from a Bunsen burner, can be used to increase digestion of keratin such that fungal hyphae can be more easily visualized. This can then be examined under light microscopy, and fungal hyphae will appear as thin, filamentous tree branches.

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 fungal culture is typically performed on Dermatophyte Test Medium (DTM). A DTM contains chloramphenicol, gentamicin, and cycloheximide to inhibit bacteria and some saprophytic fungal growth.

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. [9]

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. Hyphae typically are present at different levels of the stratum corneum and usually are parallel to the epidermis.

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