How can I diagnose a toenail fungal infection?

How to Recognize a Toenail Fungal Infection

You should suspect a toenail fungal infection if you notice thickening, discoloration (yellow, white, or brown), and separation of the nail from the nail bed, especially starting at the tip or sides of the nail, with the nail becoming brittle or crumbly. 1

Key Clinical Signs to Look For

Most Common Presentation: Distal-Lateral Subungual Onychomycosis (DLSO)

• The infection typically starts at the tip or sides of the toenail and spreads toward the cuticle, causing the nail to become thickened and discolored (yellow-brown) 1
• The nail separates from the nail bed underneath (onycholysis), though the top surface of the nail plate initially remains intact 1
• Toenails are affected far more commonly than fingernails, with the big toenail being most frequently involved 1
• The nail becomes soft and friable (crumbly) when fungal infection is present, which distinguishes it from non-fungal nail problems 1

Less Common Presentations to Recognize

• White, crumbly patches on the nail surface (Superficial White Onychomycosis) that gradually spread across the entire nail, most commonly seen in children 1
• White discoloration starting near the cuticle (Proximal Subungual Onychomycosis) and spreading toward the tip, which is uncommon in healthy individuals but more frequent in immunocompromised patients 1
• Complete nail destruction with thick, dystrophic nails in advanced cases (Total Dystrophic Onychomycosis) 1

Important Distinguishing Features

What Makes It Likely Fungal vs. Other Conditions

• Fungal infections cause the nail to become soft and friable, whereas non-infectious conditions (psoriasis, trauma, lichen planus) typically do not produce this texture change 1
• Fungal infections usually do NOT start with nail fold swelling or redness - if you see swelling around the cuticle with pain, this suggests bacterial paronychia rather than fungal infection 1, 2
• Green or black nail discoloration suggests bacterial infection (Pseudomonas aeruginosa), not fungal infection 1, 2

Risk Factors That Increase Suspicion

• Older age - onychomycosis affects 10% of the general population, 20% of those over 60, and 50% of those over 70 3
• Occlusive footwear and environments where feet remain moist 4
• Previous nail trauma or damage makes fungal infection more likely 1
• Family members with fungal nail or foot infections, as household transmission is common 4

Critical Caveat: Clinical Appearance Alone Is Insufficient

You cannot definitively diagnose fungal infection based on appearance alone - approximately 50% of dystrophic nails that look fungal are actually non-fungal conditions. 4 The British Association of Dermatologists emphasizes that laboratory confirmation is essential before starting treatment, as many conditions mimic onychomycosis including psoriasis, chronic trauma, and other nail dystrophies 1

When to Seek Medical Evaluation

• If you suspect fungal infection based on the above signs, see a healthcare provider for laboratory confirmation before starting treatment 5
• Laboratory testing (KOH preparation, fungal culture, or nail biopsy) should be obtained to confirm the diagnosis, as recommended by the FDA for terbinafine use 5
• Treatment is lengthy (3-12 months) and can have side effects, making accurate diagnosis crucial before initiating therapy 6, 3

What You Can Monitor at Home

• Progressive thickening and yellowing of the nail over weeks to months suggests active infection 1, 6
• Spread to additional nails or involvement of more than 50% of a single nail plate indicates more severe infection requiring oral rather than topical treatment 7
• Pain, discomfort, or difficulty with footwear indicates the infection is impacting quality of life and warrants treatment 6