When evaluating a rash with a Wood’s lamp, which fluorescence patterns are concerning and what diagnoses do they suggest?

Wood's Lamp Fluorescence Patterns of Concern

When evaluating a rash with Wood's lamp, coral-red fluorescence indicating erythrasma and green fluorescence suggesting Pseudomonas infection are the most concerning findings requiring immediate antimicrobial therapy, while yellow-green fluorescence in tinea versicolor and specific fluorescence patterns in tinea capitis (particularly Microsporum canis infections) warrant antifungal treatment.

Concerning Fluorescence Patterns and Their Diagnoses

Bacterial Infections (Highest Priority)

Coral-Red Fluorescence: Erythrasma

• Caused by Corynebacterium minutissimum, a gram-positive bacterium that produces porphyrins 1, 2
• Typically presents in intertriginous areas (axillae, groin) as erythematous, scaly patches
• The coral-red fluorescence is pathognomonic and allows differentiation from fungal infections like tinea versicolor or candidiasis, which lack this specific fluorescence 2
• Treatment required: Topical clindamycin or combination therapy with oral doxycycline 100mg daily for 20 days 1, 2
• Common pitfall: Often misdiagnosed as dermatophytosis, leading to inappropriate antifungal therapy 2

Green Fluorescence: Pseudomonas Infection

• Pseudomonas aeruginosa produces pyocyanin, causing green fluorescence 3, 4
• Indicates active bacterial infection requiring antimicrobial coverage
• Often occurs in wound infections, burns, or immunocompromised states

Fungal Infections

Yellow-Green Fluorescence: Tinea Versicolor

• Caused by Malassezia species 3, 4
• Presents as hypopigmented or hyperpigmented scaly patches
• Requires antifungal therapy

Fluorescence in Tinea Capitis

• Critical distinction: Only certain dermatophyte species fluoresce 5, 6
• Microsporum canis (zoophilic, ectothrix) shows bright green fluorescence under Wood's lamp 5, 6
• Favus infections (T. schoenleinii) also fluoresce 5
• Important caveat: Most common causes of tinea capitis (T. tonsurans, T. violaceum—endothrix infections) do NOT fluoresce 5
• Absence of fluorescence does NOT rule out tinea capitis
• When fluorescence is present with animal contact history, it strongly suggests M. canis infection requiring systemic antifungal therapy 6

Other Diagnostic Patterns

Blue-White Fluorescence

• Scabies burrows may show blue-white fluorescence 3
• Vitiligo shows bluish-white fluorescence, helping delineate depigmented areas in pale skin 7

White/Diamond Fluorescence

• Porokeratosis shows "diamond necklace" sign with white fluorescence of hyperkeratotic scale 3

Clinical Algorithm for Concerning Findings

1. Coral-red fluorescence in intertriginous areas:

   • Diagnose erythrasma
   • Initiate topical clindamycin or oral doxycycline
   • Avoid unnecessary antifungal therapy

2. Green fluorescence in any lesion:

   • Suspect Pseudomonas infection
   • Culture and initiate appropriate antibiotics
   • Consider underlying immunocompromise

3. Fluorescence in scalp lesions with alopecia:

   • If present with animal contact: likely M. canis tinea capitis
   • Confirm with fungal culture
   • Initiate systemic antifungal therapy
   • If absent: Still pursue fungal culture—most tinea capitis doesn't fluoresce 5

4. Yellow-green fluorescence on trunk:

   • Diagnose tinea versicolor
   • Treat with topical or oral antifungals

Critical Pitfalls to Avoid

• Do not rely solely on Wood's lamp for tinea capitis diagnosis—the majority of cases (endothrix infections) will not fluoresce 5
• Do not mistake erythrasma for fungal infection—the coral-red fluorescence is diagnostic and requires antibacterial, not antifungal therapy 1, 2
• Wood's lamp is most useful in pale skin (types I-II)—limited utility in darker skin types 7
• Always correlate fluorescence with clinical presentation—bacterial colonization of skin doesn't always indicate infection 8