Nail Fungus Cannot Be Diagnosed on Clinical Grounds Alone
Treatment should not be instituted on clinical grounds alone—mycological confirmation is mandatory before initiating antifungal therapy. 1
Why Laboratory Confirmation is Essential
Although onychomycosis has characteristic clinical features (nail thickening, yellow-brown discoloration, onycholysis), approximately 50% of all dystrophic nails are actually non-fungal in origin despite appearing clinically identical to fungal infections. 1, 2, 3
The British Association of Dermatologists explicitly states that treatment should not be prescribed without mycological confirmation because:
- Long treatment duration required: Toenails take 12 months to grow out completely, fingernails take 6 months—far too long to rely on therapeutic trial for diagnosis 1
- Treatment is not always successful: Without confirmed diagnosis, it's impossible to distinguish treatment failure from incorrect initial diagnosis 1
- Multiple non-fungal mimics exist: Psoriasis, lichen planus, trauma, yellow nail syndrome, and Darier disease can all produce identical clinical appearances 1, 2
Required Diagnostic Testing Before Treatment
Obtain mycological confirmation through one or more of the following methods:
- Potassium hydroxide (KOH) preparation with microscopy: Collect specimens from discolored, dystrophic, or brittle parts of the nail, cutting through the entire thickness including crumbly material 1, 2
- Calcofluor white staining: Significantly increases sensitivity compared to KOH alone by enhancing visualization of fungal elements 1, 3
- Fungal culture on Sabouraud's glucose agar: Identifies specific causative organism (dermatophyte vs. Candida vs. non-dermatophyte mold), which guides treatment selection 2
- Histopathological examination with periodic acid-Schiff (PAS) staining: More sensitive than direct microscopy or culture in recent studies 1
- Real-time PCR assays: Turnaround time less than 2 days, significantly increased detection rate compared to culture, though may detect dead fungus 1
Clinical Features That Suggest (But Don't Confirm) Onychomycosis
While these findings raise suspicion, they cannot replace laboratory confirmation:
- Distal and lateral subungual onychomycosis (DLSO): Most common pattern, with subungual hyperkeratosis, onycholysis, and nail plate thickening starting distally/laterally 1, 2
- Associated tinea pedis: Nearly always present with toenail dermatophyte infection 1
- Soft, friable nail texture: Suggests fungal infection versus hard, brittle texture of non-infectious causes 3, 4
- Toenails more affected than fingernails: 80% of onychomycosis cases involve toenails 1
Critical Pitfall to Avoid
Never assume fungal infection based on visual inspection alone—the nail surface in non-infectious conditions like psoriasis or lichen planus cannot be definitively distinguished from fungal infection by appearance, even though fungal infections typically produce soft, friable texture while non-infectious causes produce hard, brittle texture. 3, 4
When to Suspect Alternative Diagnoses
- Lichen planus: Approximately 10% have nail involvement with thinning of nail plate, subungual hyperkeratosis, onycholysis, and dorsal pterygium 1
- Psoriasis: Nail pitting, oil drop sign, subungual hyperkeratosis 1, 2
- Trauma: Examination of nail bed after clipping shows intact longitudinal epidermal ridges stretching to lunula (normal nail bed) 1
- Proximal subungual onychomycosis (PSO): Consider HIV, immunosuppression, peripheral vascular disease, or diabetes as underlying conditions 1