Fungal Studies: Collection Procedures and Tests

CHAPTER 19 Fungal Studies


Collection Procedures and Tests



Dennis E. Babel



Diagnostic Methods


The three basic methods used to diagnose fungal infections are direct microscopy (e.g., potassium hydroxide [KOH] method), fungal culture, and biopsy with histopathology.



Direct Microscopy


Early diagnosis of cutaneous mycoses can be made in the physician’s office by direct microscopy of infected tissue or lesion exudate. A number of different clearing solutions can be applied to collected material to assist in direct microscopy. These agents help distribute the specimen so the clinician can more readily visualize any fungal structure. These solutions include simple saline, potassium or sodium hydroxide in various formulas and preparations, and various coloring agents.



Fungal Culture


The ultimate identification of fungal pathogens requires their isolation on fungal culture medium. This isolation can take place in the physician’s office using fungal media such as Sabouraud’s dextrose agar with cycloheximide (to inhibit fungal contaminants) and chloramphenicol (to inhibit bacterial contaminants). These are commercially available as Mycosel agar (BBL Microbiology Systems, Becton Dickinson Co., Cockeysville, Md), Mycobiotic agar (Difco Laboratories, Detroit, Mich), and Dermatophyte Test Medium (DTM), a presumptive color-change medium (Hardy Diagnostics, Santa Maria, Calif). Of the three listed, only DTM is a color-change medium for purposes of this chapter. Inoculation of an appropriate patient specimen on these agars should allow the growth only of the true causative fungal organism.



Biopsy and Histopathology


Biopsy specimens obtained from fungal lesions can reveal the in vivo morphology of the infectious agent as well as the host response to this invasive presence. The appropriately stained histopathology section can provide the clinician with proof of the presence of a fungal pathogen, clues to its identity, the extent of infection, and the patient’s ability to respond to this invasion. Although this procedure might be considered the gold standard for the diagnosis of human mycoses, it is an invasive procedure, is somewhat costly, and is seldom required for the identification of cutaneous mycoses. A 3-mm punch biopsy is usually sufficient. The pathologist must be alerted if a fungal infection is considered in the differential.



Equipment


This list includes materials for collecting and examining specimens for cutaneous mycoses as described in the text. Not every item is needed for each collection method.


• Alcohol swabs

• 3 × 3 gauze squares

• Scalpel blade (no. 15)

• Toothbrush

• Cotton-tipped applicator

• Disposable biopsy punch (3 mm)

• Glass microscope slide (1 × 3 in)

• Coverglass (22 × 22 mm)

• 20% KOH with dimethyl sulfoxide solution

• Chlorazol black E solution

• Microscope with 10× and 40× objectives

• Fungal culture media in tubes, vials, or Petri dishes (see “Fungal Culture” section, earlier)


Cutaneous Mycoses Specimen Collection (for KOH Preparations and Fungal Cultures)



Hair


The most common mycoses of the hair are tinea capitis and tinea barbae (Fig. 19-1).


• Clean the area of alopecia thoroughly with alcohol to remove foreign debris and minimize bacterial contamination. This will not affect the viability of the fungi in any manner (Fig. 19-2).

• Collect a specimen with a scalpel, glass slide edge, new toothbrush, 3 × 3 gauze square, or cotton-tipped applicator (Fig. 19-3).

• Appropriate specimen could include black dots (hair stubs) or scalp scale from the area of alopecia. (Long hairs and hair clippings are unacceptable because they are seldom actually infected and are frequently contaminated with bacteria.)


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Figure 19-1 A, Black dot tinea capitis. B, Tinea capitis with kerion. C, Tinea barbae.


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Figure 19-2 Clean area with alcohol wipe.


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Figure 19-3 Tinea collection tools.



Skin


Fungal infections of the skin include tinea corporis, tinea cruris, tinea pedis, tinea manuum, and candidiasis (Fig. 19-4).


• For annular or serpiginous lesions of the skin, clean the advancing lesional edge with alcohol and obtain the scaling epithelium (avoid collecting scale from the center or oldest portion of the lesion because it is unlikely that the fungal pathogen is still present in that “healed” area). Scrape over the area firmly with the side of a scalpel blade to prevent bleeding. Loosened epithelial debris may be scraped directly onto a glass microscope slide for KOH examination and directly onto the fungal media surface for culture.

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May 14, 2017 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Fungal Studies: Collection Procedures and Tests

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