Tinea Cruris Workup

Updated: Sep 11, 2020
  • Author: Michael Wiederkehr, MD; Chief Editor: Dirk M Elston, MD  more...
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Laboratory Studies

Microscopic examination of a potassium hydroxide (KOH) wet mount of scales is diagnostic in tinea cruris. The procedure for KOH wet mount is as follows:

  • Clean the area with 70% alcohol.

  • Collect scales from the margin of the lesion; use a scalpel or the edge of a glass slide for this purpose. Cover the collected scales with a cover slip; allow a drop of KOH (10-15% wt/vol) to run under the cover slip.

  • The keratin and debris should dissolve after a few minutes. The process can be hastened by heating the slide or by the addition of a keratolytic or dimethyl sulfoxide to the KOH formulation.

  • The addition of 1 drop of lactophenol cotton blue solution to the wet mount preparation heightens the contrast and aids in the diagnosis.

  • Negative results on KOH preparation do not exclude fungal infection.

  • Scale culture is useful for fungal identification but is a more specific, albeit less sensitive, diagnostic test than KOH wet mount.

Growth on Mycosel or Sabouraud agar plates usually is sufficient within 3-6 weeks to allow specific fungal identification.



Negative KOH wet mount examination and cultures exclude other conditions in the differential diagnosis. If tinea cruris still is suggested, repeat the tests, several times if necessary.

Punch biopsy is diagnostic but has low sensitivity and low specificity. Using periodic acid-Schiff stain (fungal elements appear pink) or methenamine silver stains (fungal elements appear brown or black) on the processed tissue enhances the sensitivity of the biopsy procedure.

Wood lamp examination may be helpful to exclude erythrasma, which reveals coral red florescence of the affected area.

The images below demonstrate the appearance of tinea cruris using a variety of staining techniques.

Tinea cruris (hematoxylin and eosin stain). Tinea cruris (hematoxylin and eosin stain).
Tinea cruris (periodic acid-Schiff stain, magnific Tinea cruris (periodic acid-Schiff stain, magnification X 20).
Tinea cruris (Gomori methenamine-silver stain, mag Tinea cruris (Gomori methenamine-silver stain, magnification X 20).

Histologic Findings

Microscopic examination of hematoxylin and eosin–stained tissue sections reveals patterns of inflammation strongly suggestive of dermatophyte infection. The inflammation typically is perivascular; the epidermis exhibits spongiosis or a psoriasiform pattern of hyperplasia. Granulomatous dermatitis may accompany folliculitis.

Specific diagnostic findings include the presence of neutrophils in the cornified cell layer and the sandwich sign, in which fungal elements are sandwiched between 2 zones of differing structure within the cornified cell layer. The upper zone of the cornified cell layer has a typical basket-weave pattern of orthokeratosis, while the lower zone consists of more compact orthokeratosis and parakeratosis. The presence of spores and branching hyphae can be confirmed using periodic acid-Schiff or methenamine silver stains, but histologic examination provides no clues regarding the dermatophyte species.