Wood's Lamp Findings in Dermatophyte Scalp Infections
In most cases of tinea capitis confirmed by KOH mount, Wood's lamp examination will show no fluorescence, as the majority of current infections are caused by non-fluorescing endothrix species like Trichophyton tonsurans and T. violaceum. 1
Expected Findings Based on Causative Organism
Non-Fluorescent Infections (Most Common)
- Endothrix species (T. tonsurans, T. violaceum) do not fluoresce under Wood's lamp 1
- These organisms have become the predominant cause of tinea capitis in urban areas, particularly in the UK and North America 1
- Wood's lamp examination will be negative despite positive KOH findings 1
Fluorescent Infections (Less Common)
- Ectothrix species, particularly Microsporum canis, produce bright green fluorescence under Wood's lamp 1
- This fluorescence allows for targeted hair plucking of affected hairs for specimen collection 1
- T. schoenleinii infections (favus variant) also fluoresce under Wood's lamp 1
- Favus presents with yellow, crusted, cup-shaped lesions (scutula) and may result in scarring alopecia 1
Clinical Algorithm for Wood's Lamp Use
When to expect fluorescence:
- Geographic areas where M. canis is prevalent (zoophilic transmission from cats/dogs) 1, 2
- Patients with animal exposure history 1
- Middle Eastern or North African patients (higher T. schoenleinii prevalence) 1
When fluorescence is unlikely:
- Urban settings where anthropophilic species predominate 1
- No animal contact history 1
- Black dot pattern or kerion presentation (typically T. tonsurans) 1
Important Clinical Pitfalls
- Do not rule out tinea capitis based on negative Wood's lamp examination 1
- The absence of fluorescence does not exclude dermatophyte infection, as most contemporary cases are caused by non-fluorescing organisms 1
- Wood's lamp has limited diagnostic utility in current practice but remains useful for identifying M. canis infections when present 1, 3
- Always confirm diagnosis with KOH microscopy and fungal culture regardless of Wood's lamp findings 1