The fluorescein stain test is used by eye care professionals to evaluate the health of the eye’s surface. This diagnostic tool involves applying an orange-colored dye, fluorescein sodium, to the tear film. The dye is a fluorophore, meaning it absorbs light at one wavelength and emits it at a different, visible wavelength. When viewed under a specialized blue light, the dye highlights irregularities or areas of damage on the cornea and conjunctiva, making otherwise invisible problems readily apparent. The test is quick and fundamental for diagnosing a variety of common external eye conditions.
Conditions Diagnosed Using Fluorescein
A healthcare provider uses fluorescein staining to identify ocular surface defects that may be causing discomfort or vision issues. The test is effective at pinpointing damage to the corneal epithelium, the transparent outer layer of the eye. A common application is detecting a corneal abrasion, which is a scratch on the eye’s surface.
The stain also helps locate foreign bodies, such as dust or debris, lodged under the eyelid or on the surface of the eye. For patients with chronic discomfort, fluorescein can reveal the characteristic punctate, or dot-like, staining associated with dry eye syndrome.
The test is also used in contact lens fittings, especially for rigid gas permeable lenses, to assess the tear film layer and ensure a proper fit. In cases of suspected trauma, the dye is used in the Seidel test to check for a small leak of aqueous humor, which indicates a perforation of the globe.
Applying the Fluorescein Stain Step-by-Step
Performing the fluorescein stain begins with applying the dye to the eye’s surface. The most common method uses a sterile, single-use paper strip impregnated with dried fluorescein sodium. Before application, the strip is moistened with sterile saline solution or a topical anesthetic to create liquid dye.
The patient looks upward while the moistened strip is gently touched to the palpebral conjunctiva, the inner surface of the lower eyelid. It is important to avoid touching the sensitive cornea, as this can cause irritation or injury. Alternatively, a liquid drop of fluorescein solution may be instilled directly, sometimes pre-mixed with an anesthetic.
After application, the patient blinks several times to distribute the fluorescein across the tear film. The eye is then examined using a specialized instrument, usually a slit lamp, which allows for magnified viewing. The slit lamp uses a cobalt blue filter; fluorescein absorbs the blue light and emits a bright yellow-green light, making damaged areas visible.
Understanding the Staining Patterns
Fluorescein works by pooling where the protective outer layer of cells, the corneal epithelium, has been damaged or lost. Intact epithelial cells have tight junctions that prevent dye penetration. When these cells are compromised, the dye enters the underlying tissue, causing damaged areas to fluoresce brightly green or yellow-green under the cobalt blue light.
The pattern and location of the staining offer distinct diagnostic clues. A linear streak across the cornea suggests a corneal abrasion, often caused by a foreign object. A cluster of fine, scattered dots, known as punctate staining, is common in dry eye disease or may signal toxicity from certain eye drops.
Staining that forms a ring or annular pattern may indicate a complication related to contact lens wear. Conversely, “negative staining” occurs when a raised lesion, such as a contact lens bubble, lifts the tear film. This displaces the fluorescein, resulting in a dark, non-fluorescing area against a bright background, which helps the professional determine the nature and severity of the ocular surface issue.