Why Diagnostic Dyes Are Used
Diagnostic dyes are ophthalmic staining agents that reveal corneal and conjunctival surface abnormalities that are invisible under white light illumination. They are essential tools for detecting corneal abrasions, dry eye disease, keratoconus, contact lens-related complications, and other anterior segment conditions.
Fluorescein Sodium
Sodium fluorescein is the most widely used ophthalmic diagnostic dye. It stains areas where the corneal epithelial surface is disrupted -- it pools in any break, pit, or defect in the epithelial tight junctions. Under cobalt-blue light illumination (wavelength ~490 nm), it fluoresces brilliant green.
Uses of Fluorescein
- Corneal abrasion detection: Abrasions stain bright green. The size, depth, and location of the abrasion can be mapped.
- Goldmann applanation tonometry: Fluorescein stains the tear film to create the mire semicircles used in tonometry.
- Contact lens fitting: Fluorescein instilled under a rigid contact lens reveals the fitting pattern (where the lens touches the cornea, where it lifts, and where the tear lake is).
- Dry eye evaluation: Areas of corneal staining with fluorescein correspond to areas where the tear film has broken down and the epithelium has been exposed, indicating ocular surface disease severity.
- Tear breakup time (TBUT): The time from the last blink to the appearance of the first dark patch (fluorescein-free zone) in the tear film. Normal TBUT is more than 10 seconds; less than 5 seconds is abnormal and indicates an unstable tear film.
Forms Available
- Fluorescein strips: Paper strips impregnated with dye, wetted with sterile saline before touching to the inferior fornix. Preferred because they carry the lowest risk of contamination and are easy to control for concentration.
- Fluorescein drops (sodium fluorescein 2%): Instilled directly. Higher risk of bacterial contamination with multi-dose bottles.
- Benoxinate-fluorescein combination (Fluress): Combines topical anesthetic with fluorescein -- convenient for tonometry.
💡 Clinical Tip: Fluorescein strips should be touched to the inferior conjunctival fornix -- not the cornea. Touching the strip to the cornea can abrade the surface and create a false-positive staining result. Moisten the strip with a single drop of sterile saline first to prevent any scratching from a dry strip tip.
Rose Bengal
Rose bengal stains devitalized (unhealthy) but still-attached corneal and conjunctival epithelial cells, as well as mucus strands, red-pink. It is used to evaluate dry eye disease (keratoconjunctivitis sicca), viral keratitis (herpes simplex produces a characteristic dendritic staining pattern with rose bengal), and conjunctival abnormalities.
- Rose bengal stains are more intense and easier to see at the conjunctiva than fluorescein.
- Rose bengal is more irritating than fluorescein -- instillation can cause significant stinging. Topical anesthetic may be instilled first to improve tolerance.
- Viewed with white light (not cobalt blue). The stain appears bright red or pink.
Lissamine Green
Lissamine green has replaced rose bengal in many practices because it stains devitalized cells and mucus similarly to rose bengal but is significantly less irritating to the patient. It stains green and is viewed under white light or with red-free (green) illumination.
- Better tolerated than rose bengal -- preferred for routine dry eye assessment in sensitive patients.
- Available in strip or drop form.
Tear Breakup Time (TBUT) Test
The CPOA may perform TBUT as part of dry eye evaluation:
- Instill fluorescein using a moistened strip.
- Ask the patient to blink twice to distribute the dye.
- Ask the patient to stop blinking and look straight ahead.
- Using cobalt-blue light at the slit lamp, observe the tear film and start timing when the patient stops blinking.
- Note when the first dark dry spot (break) appears in the fluorescent tear film.
- Record the time in seconds: TBUT = __ seconds.
⚠️ Common Mistake: Using too much fluorescein for TBUT or tonometry. Excessive fluorescein creates a thick, green coat that obscures the fine mire edges needed for accurate tonometry and makes TBUT interpretation difficult. A thin, even coat from a briefly wetted strip -- not a pool of green liquid -- is ideal.
Key Takeaways
- Fluorescein stains epithelial breaks under cobalt-blue light -- used for abrasions, tonometry, contact lens fitting, and TBUT.
- Rose bengal stains devitalized cells and mucus (red) -- used for dry eye and viral keratitis (dendritic pattern).
- Lissamine green has largely replaced rose bengal -- same staining properties but much less irritating.
- Touch fluorescein strip to the inferior fornix, not the cornea; moisten the strip first.
- Normal TBUT is greater than 10 seconds; less than 5 seconds indicates an unstable tear film.
- Use thin, even fluorescein application for accurate tonometry mires and TBUT evaluation.