Diagnosis of Onychomycosis: Confirming Infection and Key Diagnostic Tests

Conclusions

• Onychomycosis should be confirmed with testing before treatment because many dystrophic nails that look fungal are not actually fungal infections.
• Routine diagnosis still relies mainly on careful nail sampling followed by KOH, PAS, culture, and sometimes PCR, with no single test being perfect.
• The highest diagnostic yield comes from submitting abundant proximal nail clippings and subungual debris, often after dermoscopic targeting of the infected area.
• Dermoscopy can improve specimen targeting and modestly increase culture sensitivity, but it does not itself confirm the organism.
• PAS on formalin-fixed nail clippings is a highly sensitive routine test, while KOH is quicker and specific but less sensitive.
• Culture is slow and relatively insensitive, so it is used mainly when non-dermatophyte molds or unusual pathogens are suspected.
• PCR is useful for equivocal, mixed, refractory, or resistance-suspected cases, but it is costly and not always covered by insurance.
• Non-dermatophyte mold onychomycosis is increasingly recognized and is harder to treat, often requiring repeated confirmation and sometimes prolonged therapy.
• Clinical clues such as proximal subungual disease, paronychia, superficial white patterns, and longitudinal streaks should raise suspicion for non-dermatophyte mold infection.
• Newer technologies such as reflectance confocal microscopy, OCT, fluorescence, and AI are promising but are not yet standard because they generally lack organism identification or viability confirmation.
• When repeated nail clippings are negative but suspicion remains high, a shallow shave biopsy of the nail surface may help diagnose infection and exclude other nail pathology.
• A complete clinical-pathologic correlation is essential because underlying nail disease, paronychia, or even neoplasm can prevent improvement if not addressed.