What's the next step for a patient with Tinea corporis (ringworm of the body) who has been taking terbinafine (antifungal medication) 250mg once daily for 14 days without improvement and is still developing new lesions?

Management of Treatment-Refractory Tinea Corporis

The next step is to obtain fungal culture to identify the causative organism, then switch to an alternative antifungal agent—specifically itraconazole or griseofulvin—while considering the possibility of terbinafine resistance or non-compliance. 1

Immediate Diagnostic Steps

• Obtain fungal culture and KOH preparation from active lesion borders to identify the specific dermatophyte species, as treatment efficacy varies significantly by organism 1
• Consider testing for terbinafine resistance if Trichophyton rubrum is isolated, as resistance mutations (particularly in the SQLE gene) are increasingly reported 2
• Assess compliance and absorption factors—terbinafine should be taken with fatty food to enhance bioavailability 1

Understanding Treatment Failure

The 14-day course is likely insufficient duration for tinea corporis. While the evidence base primarily addresses tinea capitis, the principles of treatment failure apply:

• Treatment failures occur due to: lack of compliance, suboptimal drug absorption, organism insensitivity, or reinfection 1
• Standard terbinafine courses for dermatophyte infections typically require 2-4 weeks minimum, not 14 days 1, 3
• Recent data show poor cure rates (only 20-25.9%) even with 4 weeks of terbinafine 250mg daily for tinea corporis/cruris 4

Second-Line Treatment Options

Switch to itraconazole as the preferred second-line agent:

• Itraconazole 100-200 mg daily for 2-4 weeks is effective against both Trichophyton and Microsporum species 1
• Itraconazole demonstrates superior efficacy compared to prolonged terbinafine in treatment-refractory cases 2
• SUBA-itraconazole formulations may be considered for enhanced bioavailability in resistant cases 2

Alternative: Griseofulvin for Microsporum infections:

• If culture reveals Microsporum species, switch to griseofulvin 500-1000 mg daily for 4-6 weeks 1
• Griseofulvin shows 88.5% response rates for Microsporum versus only 67.9% for Trichophyton 1

Critical Pitfalls to Avoid

• Do not simply increase terbinafine dose to 500mg—recent evidence shows no additional benefit over 250mg, with similar poor cure rates (33.3% vs 20%) 4
• Do not rely on topical therapy alone—topical agents are insufficient for established tinea corporis with ongoing new lesions 1
• Mycological cure, not clinical appearance, is the treatment endpoint—repeat cultures until clearance is documented 1, 5

Species-Specific Considerations

• Trichophyton species: Terbinafine typically shows higher efficacy, but resistance is emerging; consider itraconazole if treatment fails 1, 2
• Microsporum species: Terbinafine has poor efficacy due to inadequate tissue concentrations; griseofulvin is superior 1
• Trichophyton rubrum: Most common cause (seen in 53/64 cases in one series); consider resistance testing if refractory 6, 2

Adjunctive Measures

• Ensure complete drying of affected areas after bathing to prevent recurrence 5
• Screen household contacts for asymptomatic carriage if T. tonsurans is identified 1
• Consider topical antifungals (ciclopirox, miconazole) as adjuncts to systemic therapy, not replacements 2