Wood's Lamp Examination

Wood's lamp examination uses long wave ultraviolet light at 365 nm in a darkened room. Some organisms, porphyrins and topical substances fluoresce, and altered pigmentation shows contrast on the skin.

It is a bedside screening tool for superficial fungal and bacterial infections, porphyria, and the extent of pigment loss. Fluorescence is not specific and does not identify a species, and a negative examination does not exclude infection.

Preparation and technique. The room is darkened, and the examiner allows the eyes to adapt for several minutes. The skin is dry and free of topical agents, ointments, cosmetics and sunscreens, since these fluoresce and cause false results. Protective eyewear is used. The lamp is held close to the skin, and the lesion is examined directly and compared with normal skin on the same site.

Hair examination. In tinea capitis, infected hairs are examined under the lamp, and fluorescence indicates Microsporum infection in most cases.

Interpretation.

Limitations and artefacts. Cosmetics, sunscreens, detergents, dressings, ointments and plant material can fluoresce. Ambient light, incomplete dark adaptation and wet or greasy skin reduce contrast.

Follow up tests. Microscopy, culture or PCR confirms infection. Urine porphyrin testing confirms porphyria.

Special groups. Assessment of vitiligo is harder in darker skin, where the contrast is reduced. Children and adults with light sensitive conditions require eye protection, though exposure is low.