Diagnosis of Nail Fungus (Onychomycosis)
Laboratory confirmation through microscopy, culture, or histopathology is essential for diagnosing onychomycosis, as approximately 50% of nail dystrophies are non-fungal in origin. 1
Clinical Presentation and Initial Assessment
Onychomycosis should be suspected in patients presenting with:
- Discolored nails (white, yellow, green, or black)
- Thickened nail plate
- Subungual hyperkeratosis (buildup under the nail)
- Onycholysis (separation of nail from nail bed)
- Brittle, crumbly, or friable nail texture
- Foul-smelling nails 2
Unlike non-fungal causes of nail dystrophy, fungal infections typically make the nail surface soft and friable 1.
Diagnostic Testing Algorithm
1. Direct Microscopic Examination
- Preferred method: Calcofluor white staining (significantly increases sensitivity compared to KOH alone) 3
- Alternative: Potassium hydroxide (KOH) preparation
- Procedure: Collect nail clippings/scrapings from the most affected areas (discolored, dystrophic parts)
- Material should be taken from any discoloured, dystrophic or brittle parts of the nail 3
- For superficial white onychomycosis, take nail scrapings with a curette 3
2. Fungal Culture
- Helps identify specific pathogen (dermatophyte, yeast, or mold)
- Takes approximately 2-6 weeks for results 3
- Important for determining appropriate treatment
- Trichophyton rubrum, T. mentagrophytes, and Epidermophyton floccosum are most common dermatophytes 4
- Candida species are most common yeasts causing nail infection 3
3. Histopathology
- Periodic acid-Schiff (PAS) staining of nail clippings
- More sensitive than direct microscopy or culture 3
- Particularly useful when microscopy and culture are negative but clinical suspicion remains high
4. Molecular Diagnostic Methods
- Real-time polymerase chain reaction (PCR) assays
- Can detect and identify dermatophytes directly in nail samples
- Faster turnaround time (<2 days) compared to culture 3
- Significantly increases detection rate compared to culture 3
- Caution: May detect nonpathogenic or dead fungi 3
Differential Diagnosis
Important to rule out non-fungal causes of nail dystrophy:
- Psoriasis (pitting, oil-drop discoloration)
- Lichen planus (thinning, pterygium formation)
- Chronic trauma (distal onycholysis)
- Bacterial infections (Pseudomonas causes green/black discoloration)
- Yellow nail syndrome (yellow discoloration, hardness)
- Onychogryphosis
- Subungual malignant melanoma 1
Special Considerations
Sample Collection Tips
- Cut affected nail as far back as possible through entire thickness
- Include any crumbly material
- Nail drills, scalpels, and nail elevators may be helpful (must be sterilized between patients) 3
- If associated skin lesions are present, samples from these may yield better culture results 3
Pattern Recognition
Different clinical patterns of onychomycosis include:
- Distal and lateral subungual onychomycosis (DLSO) - most common
- Superficial white onychomycosis (SWO)
- Proximal subungual onychomycosis (PSO)
- Endonyx onychomycosis
- Total dystrophic onychomycosis (TDO) 3
Diagnostic Pitfalls
- Assuming all nail dystrophies are fungal (50% are non-fungal) 1
- Failing to obtain adequate nail samples (poor collection technique)
- Relying solely on clinical appearance without laboratory confirmation 5
- Not considering mixed infections (fungal + bacterial)
- Misinterpreting contaminants as pathogens, especially with non-dermatophyte molds 6
Remember that accurate diagnosis is crucial before initiating treatment, as antifungal therapy is lengthy, potentially has adverse effects, and should not be used unnecessarily for non-fungal nail conditions 2.