Overview
Dermatophytes are keratinophilic molds that infect skin, hair, and nails; laboratory diagnosis combines direct microscopy (KOH/calcofluor) with culture and confirmatory identification, without antifungal selection.
Classification
| Genus cue | Common lab clue | Notes |
|---|---|---|
| Trichophyton | Microconidia patterns; endothrix common in some spp. | Confirm locally; many species |
| Microsporum | Rough macroconidia; ectothrix hair patterns | Fluorescence of some hairs, not universal |
| Epidermophyton | Clubs/macroconidia; no microconidia | Skin/nail; not hair typically |
Morphologic Features
KOH: septate hyphae in keratin. Hair: ectothrix (spores outside shaft) vs endothrix (inside). Culture: cottony to powdery colonies with characteristic macro-/microconidia on LPCB mounts, species algorithms are institutional.
Laboratory Characteristics
- Extended incubation often required (policy varies)
- Cycloheximide media reduce saprophyte overgrowth, menus local
- DTM indicator change = clue only
- Non-dermatophyte molds/yeasts may grow from nails
Reference Intervals
Incubation duration, media selection, and molecular dermatophyte panel use are laboratory-specific, verify SOP rather than assuming a single universal culture day cutoff.
Clinical and Laboratory Significance
Positive KOH supports fungal elements in the specimen; culture/ID assigns organism when successful. Combined results guide clinical correlation for tinea/onychomycosis frameworks.
Differential Considerations
Differentiate dermatophytes from Candida, nondermatophyte molds, and KOH artifacts (fibers, mosaic fungus). Clinical nonfungal dystrophy can coexist with colonization.
Comparison Tables
Dermatophyte discordance cues
| Result pair | Meaning | Do not claim |
|---|---|---|
| KOH+ / culture pending | Fungal elements present | Species + drug regimen |
| KOH+ / culture− | Possible sampling/viability/prior therapy issue | Automatic “KOH artifact” without review |
| KOH− / culture+ | Culture recovered organism | KOH excluded all fungi forever |
Classification Frameworks
Not applicable.
Laboratory Notes
- Guide adequate scrapings/clippings per SOP
- Report KOH and culture as separate layers
- Never include antifungal treatment or dosing
References
Authoritative textbooks, guidelines, and reviews supporting this reference entry. Verify reference intervals, critical limits, and reflex criteria against institutional protocols and current guideline editions.
Textbooks
- Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 15th ed. Elsevier; 2020.
- Mahon CR, Lehman DC, Manuselis G. Textbook of Diagnostic Microbiology. 6th ed. Elsevier; 2018.
- Carroll KC, Hobden JA, Miller S, et al. Jawetz, Melnick, & Adelberg's Medical Microbiology. 28th ed. McGraw-Hill; 2019.
- Murray PR, Rosenthal KS, Pfaller MA. Medical Microbiology. 9th ed. Elsevier; 2021.
CLSI and Professional Guidelines
- Clinical and Laboratory Standards Institute. Performance Standards for Antimicrobial Susceptibility Testing. CLSI document M100 (verify current edition and institutional adoption).
- Centers for Disease Control and Prevention / NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL). Current edition (verify institutional adoption).
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