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Bedside Diagnostics
Potassium Hydroxide (KOH) Preparation of Scales
• Microscopic examination of scale (stratum corneum), obtained via scraping with a metal blade or glass slide and mounted in KOH, is commonly performed to confirm superficial cutaneous fungal infections (Fig. 2.1).
Fig. 2.1 Microscopic examination of potassium hydroxide (KOH) preparations of scale. A Tinea (pityriasis) versicolor due to Malassezia spp. with short mycelial forms and clusters of yeast forms. B Tinea corporis due to a dermatophyte with hyphae that cross over more than one cell (squame) and are branching. Chorazol black has been added to the KOH and the stained hyphae are easier to detect. C Branching mosaic pattern that represents the junctures of normal epidermal cells; this is a cause of false-positive KOH exams. D Cutaneous candidiasis with yeast forms and pseudohyphae. Pseudohyphae can sometimes be difficult to distinguish from hyphae. A, Courtesy, Ronald Rapini, MD; C, Courtesy, Louis Fragola, MD; D, Courtesy, Frank Samarin, MD.
• These fungal infections include tinea (pityriasis) versicolor, tinea corporis/faciei/manuum/cruris/pedis, and cutaneous candidiasis (see Chapter 64 on fungal diseases).
• Addition of chlorazol black to the KOH can improve detection (see Fig. 2.1B).
• For onychomycosis, both nail plates and subungual debris are examined; in addition, nail plates can be fixed in formalin and stained with periodic acid–Schiff (PAS) or Gomori methenamine silver stain (see Chapter 64).
Potassium Hydroxide (KOH) Preparation of Hair Shafts
• Tinea capitis is divided into two major forms: (1) endothrix – conidia occur within the hair shaft; and (2) ectothrix – while the fungus grows inside the hair shaft, conidia form on its surface (Figs. 2.2 and 2.3; see Chapter 64).
Fig. 2.2 The three patterns of hair invasion in tinea capitis and the causative dermatophytes. See Chapter 64 for additional details.
Mineral Oil Scraping for Suspected Scabies
• Place 2–3 drops of mineral oil on a glass slide. Dip the metal blade into the oil and then scrape suspicious lesions, e.g. burrows, inflammatory papules. Next place skin scrapings on a glass slide. Several skin lesions should be scraped. Dermoscopy can be used to better identify burrows and an adult female mite at the end of the burrow prior to scraping (see Chapter 71). Sometimes, KOH is used rather than mineral oil. Firm application of transparent adhesive tape to suspicious lesions followed by rapid removal and transfer to a glass slide is another technique that can be used, providing easier transport to a laboratory.
• In addition to adult mites, eggs and feces (scybala) can be seen when scrapings are examined microscopically (Figs. 2.4 and 2.5).
Fig. 2.4 Microscopic examination of scrapings from a patient with scabies. Female Sarcoptes scabiei var. hominus mite with eggs. There is a flattened, oval body with wrinkle-like corrugations and eight legs. With permission from Taplin D, Meinking TL. Infestations. In: Schachner LA, Hansen RC (Eds.), Pediatric Dermatology, 4th edn. Edinburgh, UK: Mosby, 2011:1141–1180.
