How to differentiate between fungal and bacterial toenail infections?

Differentiating Fungal from Bacterial Toenail Infections

Laboratory confirmation with potassium hydroxide (KOH) preparation and fungal culture is essential before initiating treatment, as clinical appearance alone is insufficient to distinguish fungal from bacterial infection—approximately 50% of dystrophic nails are non-fungal despite similar presentation. 1, 2

Clinical Distinguishing Features

Fungal Infection Characteristics

• Nail texture becomes soft and friable with thickening, yellow-brown discoloration, and varying degrees of onycholysis (nail separation from bed) 1, 3
• Toenails are predominantly affected (80% of dermatophyte infections), with distal and lateral subungual pattern being most common 1, 4
• Subungual hyperkeratosis (debris under nail) is typically prominent 1, 4
• The nail surface becomes crumbly and brittle in advanced cases 4
• Pressure on and movement of the nail is typically painless in dermatophyte infections 1

Bacterial Infection Characteristics

• Green or black discoloration is pathognomonic for Pseudomonas aeruginosa infection (Green Nail Syndrome) 1, 2, 5
• The nail surface typically does not become soft and friable as in fungal infection 1, 3
• Often associated with paronychia (nail fold swelling, erythema, and pain) 1, 3
• Bacterial superinfection commonly coexists with fungal infection, making differentiation challenging 1

Candida Infection (Yeast) Characteristics

• Begins in the proximal nail plate rather than distally, unlike dermatophyte infections 1
• Paronychia is nearly always present with swollen, erythematous, painful periungual skin 1
• Fingernails are more commonly affected than toenails, particularly in patients with wet occupations 1
• Pressure on and movement of the nail is painful, unlike dermatophyte infections 1
• A prominent gap develops between the nail fold and nail plate 1

Diagnostic Algorithm

Step 1: Clinical Assessment

• Examine nail location: Toenail involvement suggests dermatophyte; fingernail involvement with wet occupation suggests Candida 1
• Assess nail fold: Presence of paronychia (swelling/erythema) strongly suggests Candida or bacterial infection 1, 3
• Evaluate nail color: Green/black discoloration indicates Pseudomonas; white marks in proximal/lateral portions suggest Candida 1, 2
• Test nail texture: Soft and friable indicates fungal; hard surface suggests non-infectious or bacterial causes 1, 3
• Check for pain: Painful nail movement suggests Candida or bacterial infection rather than dermatophyte 1

Step 2: Laboratory Confirmation (Mandatory)

• Direct microscopy with KOH preparation to visualize fungal elements 2, 4
• Fungal culture on Sabouraud's glucose agar to identify specific organism 2, 4
• Calcofluor white staining enhances visualization of fungal elements 2, 4
• Obtain specimens from discolored, dystrophic, or brittle nail parts, cutting through entire thickness including crumbly material 1

Step 3: Examine for Associated Findings

• Check for concomitant tinea pedis or tinea capitis in the patient 2
• Examine family members for onychomycosis and tinea pedis, as household transmission is common 2
• Look for associated skin lesions that may harbor the same organism 1

Critical Pitfalls to Avoid

• Never diagnose fungal infection based on appearance alone—50% of dystrophic nails are non-fungal despite similar clinical presentation 1, 2, 4
• Do not overlook bacterial superinfection in fungal infections, as it commonly coexists and may require separate treatment 1
• Avoid assuming single pathogen—approximately 50% of suspicious toenails contain both fungi and bacteria 6
• Do not miss non-infectious mimics including psoriasis, lichen planus, trauma, and yellow nail syndrome, which can present similarly but require different management 1, 2
• Never initiate systemic antifungal therapy without mycological confirmation, as treatment duration is prolonged (12 weeks for toenails) and unnecessary exposure carries risks 2, 7

Treatment Implications Based on Etiology

Confirmed Fungal (Dermatophyte) Infection

• Terbinafine is first-line: 250 mg daily for 12 weeks for toenails, 6 weeks for fingernails, with 70% mycological cure rate 2, 7
• Itraconazole pulse therapy: 5 mg/kg/day for 1 week per month for 3 pulses (toenails) or 2 pulses (fingernails) 2

Confirmed Candida Infection

• Azoles are preferred over terbinafine for Candida species 2
• Address underlying moisture exposure and occupational factors 1

Confirmed Bacterial Infection (Pseudomonas)

• Keep affected area dry and apply topical povidone iodine 2% twice daily 2, 3
• Oral ciprofloxacin for severe or refractory cases 5