Laboratory diagnosis of onychomycosis can:
(1) confirm the diagnosis, especially when mixed with other nail disorders
(2) identify the causative fungus
(3) allow for drug susceptibility testing to antifungal agents
(4) document cure
(5) differentiate relapse from exposure to a different fungus
Modalities:
(1) microscopic exam of scraping or nail biopsy
(2) culture
(3) PCR or molecular methods
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Modality
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Advantages
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Disadvantages
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microscopy
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rapid, potentially permanent evidence
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insensitive when the number of organisms is low, often cannot determine species
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Culture
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allows speciation, drug susceptibility testing possible
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slow, insensitive, subject to sampling error, requires viable organisms
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molecular method
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rapid
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may detect a contaminant, can be expensive, may require multiple probes due to large number of fungal species involved, cannot do drug susceptibility studies
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For a dermatophyte, a single culture isolate of a dermatophyte is sufficient, especially when supported by microscopic evidence of onychomycosis.
For a non-dermatophyte yeast or fungus, diagnosis is more challenging because of the high frequency of contaminants.
Walshe and English criteria for a non-dermatophyte onychomycosis:
(1) a fungus should be both seen on microscopy and isolated in culture
(2) the same fungus species must be isolated in >= 5 inocula (after cutting a specimen cut into many pieces and placing each on agar media)
(3) no dermatophyte isolated in culture
Gupta et al added isolation of the same non-dermatophyte fungus on more than 1 occasion.
The criteria of Walsh and English can lead to false positive and false negative diagnoses, especially when considering the number of fungal isolates in culture.
