Onychomycosis Diagnosis
Laboratory confirmation through microscopy and culture is mandatory before initiating any treatment, as incorrect diagnosis is the most common cause of treatment failure. 1, 2
Essential Diagnostic Approach
Clinical Assessment
- Look for specific nail changes: distal nail thickening, yellow-brown discoloration, subungual debris, onycholysis (nail separation), and friable nail texture that distinguishes fungal infection from other nail dystrophies 1
- Examine for dermatophytoma: dense white lesions beneath the nail (tightly packed hyphae in subungual space), most common in great toenails, which predicts treatment resistance 1
- Assess nail thickness: measure if >2mm, as this correlates with treatment failure 1
- Identify the pattern: distal-lateral subungual onychomycosis (most common), superficial white onychomycosis, proximal subungual, or total dystrophic 1
Differential Diagnosis to Exclude
- Psoriasis: nail pitting, oil drop sign, but nail surface remains hard (not soft/friable like fungal infection) 1
- Trauma: examine nail bed for intact longitudinal epidermal ridges stretching to lunula (indicates trauma, not infection) 1
- Candida paronychia: proximal nail fold inflammation with nail plate involvement, more common in fingernails of women with water exposure 1
- Bacterial infection (Pseudomonas): green or black discoloration, can coexist with fungal infection 1
Laboratory Confirmation (Mandatory)
Standard Testing
- Collect specimen properly: clip affected nail, scrape subungual debris from nail bed, include material from proximal edge of onycholysis 1
- Direct microscopy with calcofluor white: superior to potassium hydroxide (KOH) for visualizing fungal elements, increases sensitivity 1
- Fungal culture: requires 2-6 weeks, identifies specific organism (dermatophyte vs. non-dermatophyte mold vs. yeast), essential for treatment selection 1
- Both tests required: microscopy confirms fungal presence, culture identifies species 1, 2
Advanced Diagnostic Options
- Histopathology with PAS staining: more sensitive than microscopy or culture, but not widely available in most clinics 1
- Real-time PCR: turnaround <2 days, higher detection rate than culture, but may detect dead/non-pathogenic fungus limiting clinical utility 1
- Use PCR selectively: helpful when culture negative but clinical suspicion high, or for treatment failure cases 1
Critical Pitfalls to Avoid
- Never treat based on clinical appearance alone: this is the leading cause of treatment failure in clinical practice 1, 2
- Beware of non-dermatophyte molds: require multiple positive cultures to confirm pathogenicity (they are common contaminants) 3
- Don't miss coexisting conditions: bacterial superinfection may require separate treatment 1
- Recognize Candida patterns: proximal nail involvement with paronychia suggests Candida (5-10% of cases), not dermatophyte 1
Documentation Before Treatment
Prior to initiating any systemic therapy, confirm: 4
- Positive KOH preparation OR
- Positive fungal culture OR
- Positive nail biopsy
For systemic therapy candidates, obtain baseline: 1, 5, 2
- Liver function tests
- Complete blood count
- Document nail involvement percentage and number of affected nails