Hydroxyxanthine dyes such as fluorescein have been used for decades to aid assessment of the ocular surface, especially the cornea. Fluorescein is available topically in eye drops (Fluorescein Sodium 1% or 2% minims) or impregnated strips. The strips can be moistened with a drop of saline, or topical anaesthetic, the patient asked to look up and fluorescein released onto the ocular surface by gently touching the inside of the lower eyelid (making sure not to abrade the cornea).
This water soluble dye readily enters and stains the corneal stroma in areas where the epithelium is absent (or even when the epithelial cells have loose intercellular junctions). Fluorescein is, therefore, extremely useful for detecting areas of epithelial deficiency that occur, for example, in corneal abrasions and herpes simplex keratitis. Although fluorescein staining can be seen using a white light, it is best detected with cobalt-blue filter illumination which causes the dye to fluoresce.