Wood's Lamp in Clinical Dermatology
Examination Protocols, Bacterial & Fungal Fluorophores & Differential Diagnosis Charts
A Wood's lamp examination is a non-invasive diagnostic technique utilizing filtered 365 nm UV-A light to evaluate superficial cutaneous infections, pigmentary abnormalities, and porphyria.
Standardized Clinical Protocol: The examination room must be completely darkened (windowless or blacked-out blinds). The clinician must dark-adapt for at least 3–5 minutes. The Wood's lamp is held 10–15 cm (4–6 inches) from the patient's skin. The skin must not have been washed within the preceding 24 hours, and topical creams, deodorants, or makeup must be avoided as they introduce false-positive fluorescence.
1. Master Diagnostic Fluorescence Chart
The table below provides pathognomonic fluorescence colors, metabolic fluorophores, and diagnostic rules:
| Clinical Condition | Pathogen / Etiology | Wood's Fluorescence Color | Responsible Fluorophore | Clinical Utility & Differential Diagnosis |
|---|---|---|---|---|
| Vitiligo | Autoimmune destruction of epidermal melanocytes; complete absence of melanin pigment | Brilliant, stark chalky blue-white with sharply demarcated borders |
Auto-fluorescence of dermal collagen fibers uncovered by complete absence of epidermal melanin absorption | Crucial for identifying subclinical depigmentation patches before they become visible to naked eye; differentiates vitiligo from post-inflammatory hypopigmentation (which appears dull off-white with fuzzy margins) |
| Tinea Capitis (Scalp Ringworm) | Fungal dermatophyte infection of hair shaft by Microsporum species (M. canis, M. audouinii, M. ferrugineum) | Brilliant electric yellow-green or bright apple-green along hair shafts |
Pteridine metabolite synthesized by Microsporum fungi within ectothrix hair infections | Differential diagnostic breakthrough: Trichophyton species (e.g., T. tonsurans) DO NOT fluoresce under Wood's lamp. Green fluorescence confirms Microsporum etiology and guides direct epilation for fungal culture. |
| Tinea Versicolor (Pityriasis Versicolor) | Superficial fungal infection of stratum corneum by Malassezia yeasts (Malassezia globosa / furfur) | Warm golden-yellow or coppery-orange fluorescence |
Pityriacitrin (bis-indole alkaloid synthesized by Malassezia in presence of sebum) | Distinguishes active Malassezia infection from residual post-inflammatory hypopigmentation; evaluates response following topical antifungal therapy |
| Erythrasma | Superficial bacterial infection of intertriginous skin by Corynebacterium minutissimum | Unmistakable striking coral-red to pinkish-orange fluorescence |
Coproporphyrin III synthesized endogenously by Corynebacterium minutissimum | Prevents catastrophic diagnostic error: Erythrasma in the groin or toe webs is frequently misdiagnosed as tinea cruris (jock itch) or candidiasis. Coral-red glow clinches diagnosis of bacterial erythrasma, requiring erythromycin or clindamycin rather than antifungals. |
| Pseudomonas aeruginosa Infection | Gram-negative opportunistic bacterial infection in burn wounds, ulcers, and toe webs | Distinct bright lime-green / yellowish-green fluorescence |
Pyoverdine (water-soluble fluorescent siderophore produced by Pseudomonas species) | Detects early Pseudomonas colonization in burn units before overt sepsis or purulent green discharge develops; guides rapid topical silver or polymyxin treatment |
| Acne Vulgaris & Folliculitis | Colonization of pilosebaceous follicles by Cutibacterium acnes (formerly Propionibacterium acnes) | Punctate, pinpoint orange-red to bright coral fluorescence within follicular orifices |
Endogenous protoporphyrin IX and coproporphyrins produced by Cutibacterium acnes metabolism of sebum | Quantitative assessment of follicular bacterial load in cosmetic dermatology and clinical trials; monitors reduction in C. acnes following photodynamic therapy or topical retinoids |
| Melasma (Epidermal vs. Dermal) | Acquired hypermelanosis of sun-exposed areas; excess melanin deposition in epidermis vs dermis | • Epidermal melasma: Pigmentation contrast is dramatically accentuated (darker and more distinct)
• Dermal melasma: Pigmentation contrast does NOT accentuate (remains fuzzy and subtle)
• Mixed melasma: Uneven patchy accentuation |
Melanin acts as a broadband UV absorber; superficial epidermal melanin blocks dermal collagen fluorescence efficiently, sharpening contrast | Essential for treatment prognosis: Epidermal melasma responds favorably to topical hydroquinone, azelaic acid, and chemical peels; dermal melasma responds poorly and requires specialized non-ablative laser therapies |
| Porphyria Cutanea Tarda (PCT) | Inborn error of heme biosynthesis: deficiency of uroporphyrinogen decarboxylase leading to systemic accumulation of uroporphyrins | Urine sample fluoresces vivid pink-orange to brilliant ruby-red under Wood's lamp (acidification with 10% HCl enhances fluorescence) |
Uroporphyrin and 7-carboxylate porphyrin in patient's urine | Rapid bedside screening test in emergency or clinical settings for patients presenting with blistering, skin fragility, and facial hypertrichosis |
2. Critical Clinical Pitfalls & Rules
Differential Note: Erythrasma vs. Tinea Cruris
Erythrasma in the groin or axilla is frequently misdiagnosed as tinea cruris (fungal jock itch) or candidiasis. Prescribing antifungals fails to resolve the infection.
Under Wood's lamp, Corynebacterium minutissimum glows unmistakable brilliant coral-red due to coproporphyrin III. This clinches the bacterial diagnosis instantly, indicating oral or topical erythromycin/clindamycin.
Diagnostic Note: Microsporum vs. Trichophyton
Wood's lamp only identifies scalp ringworm caused by Microsporum species (M. canis, M. audouinii), which glow bright yellow-green due to pteridine.
Infections caused by Trichophyton species (including T. tonsurans, the dominant cause of tinea capitis in North America) DO NOT FLUORESCE. A negative Wood's lamp examination never rules out fungal tinea capitis; KOH preparation and fungal culture remain necessary.