Dermatophytosis laboratory findings

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Laboratory Findings

Specimen collection

Scraping should be collected from active margin and transported in a presterilized black chart paper which keeps the specimen dry thus, preventing over growth of bacterial contaminants.

Direct microscopic examination

• Treatment of skin specimen with 10–20 percent potassium hydroxide (KOH) is a convenient bedside tool to provide evidence of dermatophytic infection.
• Positive scrapings are characterized by:
  • Refractile, long, smooth, undulating, branching, and septate hyphal filaments with or without arthroconidiospores.
• Fluorescent staining of the cell wall is the most sensitive method to microscopically detect fungi in skin scales as well as in specimens from nails and hair.

Culture and antifungal sensitivity

• Sabouraud dextrose agar (SDA, 4% peptone, 1% glucose, agar, water) is the most commonly used isolation media for dermatophytosis and serves as the medium for fungal growth.
• Development of colony takes 7–14 days.
• Modified SDA, with addition of gentamicin, chloramphenicol and cycloheximide is more selective for dermatophytes as chroramphenicol inhibits the growth of saprophytic fungus.
• Dermatophyte test medium is an alternative to isolation media that contain pH indicator phenol red. It is incubated at room temperature for 5–14 days.
• Dermatophytes utilize the protein resulting in excess ammonium ion and alkaline environment which turn the medium from yellow to bright red.

Hematoxylin and eosin staining

• Histology may be used in diagnosis of Majocchi's granuloma in which KOH examination of scale may be false negative.
• Hyphae may be visualized in stratum corneum on hematoxylin and eosin staining.
• Special stains most commonly used are:
  • Periodic acid-Schiff
  • Gomori methanamine silver

Dermoscopy

• Tinea capitis may show:
  • Comma hairs, which are slightly curved, fractured hair shafts
  • Corkscrew hair shave.
  • Broken hair
• Tinea corporis may show:
  • Involvement of vellus hair

Polymerase chain reaction and nucleic acid sequence based amplification

These tests not only help in the rapid and early diagnosis of infection but also help in determining drug resistance, and include:

• Uniplex PCR for direct dermatophyte detection in clinical samples: A PCR for the direct detection of dermatophytes in skin scales is available as in-house PCR-ELISA assay which separately identifies numerous dermatophyte species.
• Multiplex PCR for fungal detection in dermatophytes: Commercially available multiplex PCR tests enable simultaneous amplification of 21 dermatomycotic pathogens with subsequent DNA detection by means of agarose gel electrophoresis.