Diagnostic tools
Wood’s light
examination of the eye after fluorescein staining. (A low-cost, small ultraviolet light unit called ‘the black light’ is also available.)
Simply hold the ultraviolet light unit above the area for investigation in a dark room.
Skin conditions that produce fluorescence in Wood’s light
| Tinea capitis | Green/bright yellow (in hairs) |
| Erythrasma | Coral pink |
| Pityriasis versicolor | Pink–gold |
| Pseudomonas spp. | Yellowish-green |
| Porphyria cutanea tarda | Red (urine) |
- Not all cases of tinea capitis fluoresce because some species that cause the condition do not produce porphyrins as a by-product
- Porphyrins wash off with soap and water, and a negative result may occur in a patient who has shampooed the hair within 20 hours of presentation.
- Consequently, a negative Wood’s light reading may be misleading.
- The appropriate way of confirming the clinical diagnosis is to send specimens of hair and skin for microscopy and culture.
Skin scrapings for dermatophyte diagnosis
- Clinical indications:
- tinea (superficial dermatophyte infection)
- pityriasis versicolor
- Candida