I have a chronic mildly itchy reddish‑brown patch under my breasts—could it be erythrasma or tinea corporis, and how can I differentiate and treat it?

Differentiating and Treating Erythrasma vs Tinea Corporis Under the Breasts

This chronic, mildly itchy reddish-brown patch under your breasts is most likely erythrasma, and you should confirm the diagnosis with Wood's lamp examination showing coral-red fluorescence, then treat with topical clindamycin or mupirocin for 2 weeks. 1, 2

Key Diagnostic Approach

Wood's Lamp Examination (First-Line Diagnostic Tool)

• Perform Wood's lamp examination immediately to differentiate between these conditions—this is the single most useful diagnostic test 1, 2, 3
• Erythrasma produces bright coral-red or coral-pink fluorescence due to porphyrins released by Corynebacterium minutissimum 1, 2, 3
• Tinea corporis does not fluoresce under Wood's lamp (except rare Microsporum canis infections, which show green fluorescence and are uncommon in this location) 4

Clinical Features That Distinguish These Conditions

Erythrasma characteristics:

• Reddish-brown to hyperpigmented patches with fine scaling 1, 2
• Minimal to mild itching (less pruritic than fungal infections) 1
• Well-demarcated borders with central hypopigmentation possible 2
• Commonly affects intertriginous areas including inframammary folds 1, 5
• Associated with warm, moist environments, obesity, and diabetes 1

Tinea corporis characteristics:

• More intensely pruritic than erythrasma 5
• Active, raised, scaly border with central clearing (classic "ringworm" appearance) 6
• Erythema with varying degrees of inflammation 6

Confirmatory Testing if Wood's Lamp is Negative or Unavailable

• KOH preparation of skin scrapings: Shows hyphae/arthroconidia in tinea, negative in erythrasma 1, 5
• Gram stain: Shows gram-positive filamentous rods (Corynebacterium) in erythrasma 5, 3
• Note: Culture of Corynebacterium is difficult, often unsuccessful, and unnecessary for diagnosis 5, 3

Important Clinical Caveat

Both conditions can coexist in the same patient—studies show 62.5% of erythrasma cases have concurrent fungal infections with Candida or dermatophytes 5. If Wood's lamp is positive for erythrasma but symptoms persist after appropriate treatment, perform KOH examination to rule out concurrent fungal infection 5.

Treatment Algorithm

For Confirmed Erythrasma:

First-line treatment:

• Topical clindamycin 2% ointment twice daily for 2 weeks leads to complete resolution 1
• Alternative: Topical mupirocin 2% ointment twice daily for 2 weeks 2
• These topical treatments are more convenient and equally effective 1, 2

For Confirmed Tinea Corporis:

• Topical antifungal therapy (azoles or allylamines) for 2-4 weeks 6
• Oral antifungals (terbinafine or griseofulvin) reserved for extensive or refractory cases 6, 7

For Concurrent Infections:

• Treat both conditions simultaneously: topical antibacterial for erythrasma plus topical antifungal for tinea 5

Prevention of Recurrence

• Keep inframammary area dry and clean 1
• Address predisposing factors: control diabetes, reduce moisture accumulation 1
• Avoid sharing personal items that contact affected areas 7