Does Ringworm Fluoresce Under Black Light?
Most ringworm infections do NOT fluoresce under a Wood's lamp (black light), making this an unreliable diagnostic tool for the majority of cases. 1, 2
Fluorescence Patterns by Organism
Only specific dermatophyte species produce fluorescence under ultraviolet light:
- Microsporum canis shows green fluorescence and is one of the few dermatophytes reliably detected by Wood's lamp 1, 3
- Favus (a variant of tinea capitis) demonstrates fluorescence 1
- Trichophyton tonsurans, the most common cause of tinea capitis in athletic populations and the United States, produces white fluorescence rather than green, making it less distinctive 3
- Most common dermatophytes do NOT fluoresce, including many Trichophyton species 1
Clinical Utility and Limitations
Wood's lamp examination has limited application for the majority of ringworm cases because:
- The British Association of Dermatologists notes that Wood's lamp is primarily useful only for identifying M. canis infections and favus 1, 2
- Clinical diagnosis without laboratory confirmation is unreliable, as other conditions (eczema, psoriasis, atopic dermatitis) can mimic tinea infections 1, 4
- Treatment success should never be determined by Wood's lamp findings - mycological cure (negative KOH preparation or culture) is the definitive endpoint 2
Recommended Diagnostic Approach
Instead of relying on Wood's lamp:
- KOH preparation showing hyphae or arthroconidia provides rapid confirmation 1
- Fungal culture on Sabouraud agar is the gold standard for species identification 1
- Laboratory confirmation is essential because clinical appearance alone is unreliable 1, 4
- Collect specimens using a blunt scalpel from the active border of lesions 2