Fluorescein

Mechanism of Action

As an ocular disclosing agent, fluorescein sodium dissolves readily in aqueous alkaline solutions, responds to cobalt blue light at 465 to 490 nm, and fluoresces bright green at 520 to 530 nm. The compound contains a conjugated system that fluoresces when electrons remain in an excited state for an extended period. Fluorescein is often administered in combination with a local anesthetic such as proparacaine or benoxinate, which inhibits sodium ion channels, stabilizes nerve cell membranes, and inhibits nerve impulse conduction. When fluorescein is used to stain cells, its charged ends interact with the hydrophilic regions of the cell membrane, forming strong electrostatic bonds. One or more of the charged ends of fluorescein may then be modified to enhance molecular interactions.[12]

Pharmacokinetics

Distribution: Fluorescein distributes into the interstitial space, with an estimated volume of distribution of 0.5 L/kg.

Metabolism: Fluorescein is rapidly metabolized to fluorescein monoglucuronide. After IV administration, approximately 80% of plasma fluorescein is converted to the glucuronide conjugate within 1 hour.

Excretion: Fluorescein and its metabolite are primarily eliminated through renal excretion. After IV administration, urine may remain slightly fluorescent for 24 to 36 hours, and systemic clearance is generally complete within 48 to 72 hours after a 500 mg dose.

Administration

Available Dosage Forms and Strengths

Fluorescein is available as an IV solution and as ophthalmic strips.

• IV solutions are available as Fluorescite 10%, AK-FLUOR (preservative-free) 10%, and generic fluorescein sodium in 10% (5 mL) and 25% (2 mL) formulations.
• Ophthalmic strips are available as Bio Glo, Fluor-I-Strips A.T., GloStrips (orange flavor), and generic fluorescein sodium strips, each containing 1 mg of fluorescein sodium.

Adult Dosage

Ophthalmic administration of fluorescein begins with a paper strip with one tip stained with fluorescein. The paper strip is moistened with saline water and then placed on the conjunctiva or inferior fornix. The patient may blink a few times to spread the dye across the eye.

IV fluorescein sodium is administered at the antecubital vein (rate of administration is approximately 1 mL/s). The normal adult dose of AK-FLUOR 10% is 5 mL (500 mg) administered intravenously.

The dye appears almost immediately in the retina and choroidal vessels (in 7 to 14 s). Given the risk of anaphylaxis, IV access should be established for epinephrine administration. After fluorescein administration, the examiner can visualize fluorescence under cobalt blue light, often with reduced ambient lighting.[15]

For ocular foreign body removal, the clinician should instill 1 or 2 drops into each eye prior to the procedure.

Fluorescein solution may be administered orally and takes 10 to 15 minutes to appear. Although the solution is bitter, combining it with sugar or a beverage can increase palatability. (This represents an off-label use.)

Specific Patient Populations

Hepatic impairment: The manufacturer’s labeling does not specify any dose adjustments for patients with hepatic impairment.

Renal impairment: The manufacturer’s labeling does not specify any dose adjustments for patients with renal impairment, as minimal systemic exposure generally eliminates the need for dose modification.

Pregnancy considerations: Animal reproduction studies evaluating fluorescein sodium are lacking, and its potential to cause fetal harm has not been established. Use during pregnancy should be limited to situations where the expected benefit justifies the potential risk.

Breastfeeding considerations: Fluorescein sodium is excreted in human breast milk; therefore, caution is advised when administering it to breastfeeding individuals. Following IV administration, fluorescein concentrations in breast milk are higher than those observed after ophthalmic use, resulting in greater potential infant exposure. Although these levels are generally not expected to cause harm in most infants, caution is advised. Temporary avoidance of intense light exposure, such as phototherapy, may be considered for a few days after maternal administration.[16]

Systemic exposure following ophthalmic administration of fluorescein is minimal; therefore, adverse effects in breastfed infants are unlikely. To further reduce drug transfer into breast milk after ophthalmic use, patients may apply gentle pressure over the nasolacrimal duct for at least 1 minute and blot excess solution with absorbent tissue.

Pediatric patients: Clinical studies have included pediatric patients, with no meaningful differences observed in safety or efficacy compared with adults. For children, the AK-FLUOR dose should be calculated as 35 mg per 4.54 kg of body weight (7.7 mg/kg).

Adverse Effects

The most common adverse effect of the injectable solution is temporary discoloration of the skin and urine. The skin may develop a transient yellowish hue, while the urine may appear bright yellow. Skin discoloration typically resolves within 6 to 12 hours, whereas urine discoloration generally clears within 24 to 36 hours. Nausea is the second most frequently reported adverse reaction. Vomiting and other gastrointestinal symptoms may also occur, and some patients may experience a transient, strong, or unpleasant taste following injection.

Hypersensitivity reactions have been reported, including pruritus, generalized urticaria, bronchospasm, and, in rare cases, anaphylaxis.[15] Cardiopulmonary events such as hypotension and syncope may occur, while serious outcomes, including basilar artery ischemia, severe shock, cardiac arrest, and death, have been reported rarely. Neurologic effects include headache, with rare cases of convulsions reported following injection. Thrombophlebitis at the injection site has also been observed, and extravasation may cause intense local pain accompanied by a dull, aching sensation in the affected limb.

Other commonly reported adverse reactions include a change in taste (oral use), eye discoloration (ophthalmic), severe eye stinging and irritation (ophthalmic), and extravasation at the injection site.

Drug-Drug Interactions

Fluorescein has no known significant drug interactions.

Contraindications

Hypersensitivity to fluorescein formulations contraindicates their use. Furthermore, although additional research is needed to evaluate fetal effects, pregnancy may be considered a relative contraindication. However, based on the drug's pharmacology and limited systemic absorption, fetal harm is not expected.[15] Likewise, the drug may be used during breastfeeding despite the absence of human data, as no harm to the infant or impact on milk production is anticipated based on pharmacological properties.

If the formulation includes an ester anesthetic, as is often the case, hypersensitivity to these agents also constitutes a contraindication. Other contraindications include sensitivity to para-aminobenzoic acid and prolonged use.

Warnings and Precautions

Reported respiratory reactions warrant caution in patients with a history of allergy or bronchial asthma; appropriate emergency equipment should be readily available. In cases where hypersensitivity is suspected, intradermal skin testing (0.05 mL) may be considered prior to IV administration, although a negative result does not reliably exclude the possibility of an allergic reaction.

Severe local tissue injury may result from extravasation during injection, likely related to the alkaline nature of fluorescein solution. Reported complications include skin sloughing, superficial phlebitis, subcutaneous granuloma formation, and toxic neuritis, particularly along the median nerve in the antecubital region. These reactions may be associated with significant pain that can persist for several hours. If extravasation occurs, administration should be stopped immediately, and appropriate supportive measures should be initiated to manage tissue injury and alleviate pain.

Monitoring

During ophthalmic examination, contact lenses should be removed before fluorescein administration to avoid unnecessary staining.[2] Fluorescein is completely cleared from the urine within 2 to 3 days after injection.[15] No routine laboratory tests or monitoring parameters are recommended. Patients should be monitored for respiratory reactions and for extravasation at the injection site.

Toxicity

Fluorescein is generally well tolerated during diagnostic procedures; however, serious hypersensitivity reactions, including rare cases of anaphylaxis, have been reported. Patients with a history of allergy or bronchial asthma should be monitored carefully, and emergency equipment should remain immediately available. If hypersensitivity is suspected, intradermal skin testing may be considered before IV administration, although a negative test does not exclude the possibility of an allergic reaction. Fluorescein sodium is excreted in human milk; therefore, caution is advised when administering it to breastfeeding patients.[17]

Enhancing Healthcare Team Outcomes

Fluorescein is used primarily as a diagnostic adjunct by ophthalmologists, surgeons, and optometrists, although general physicians, nurse practitioners, and physician assistants may also use it in appropriate clinical settings. Safe and effective use requires coordinated interprofessional collaboration. Physicians and advanced practitioners are responsible for patient selection, confirming indications, assessing allergy or asthma history, obtaining informed consent, and interpreting results to guide subsequent management. Pharmacists support safe use by verifying the correct formulation and concentration, screening for contraindications or hypersensitivity risks, and ensuring proper storage and handling of the product.

Nurses and trained medical assistants play a central role in preparation and administration by confirming the correct dose and route, maintaining aseptic technique, and ensuring readiness of emergency equipment. They are also responsible for continuous patient monitoring during and after administration, including the prompt recognition and escalation of adverse reactions such as hypersensitivity, cardiopulmonary events, and extravasation-related injury. Clear communication among team members is essential, particularly when unexpected reactions occur, to enable rapid intervention and minimize harm.

From a systems and ethics perspective, all team members share responsibility for patient-centered care, including clear patient counseling about expected effects (eg, temporary discoloration), potential risks, and postprocedure expectations. Documentation of administration details, observed reactions, and follow-up plans is critical for continuity of care. Effective interprofessional collaboration through defined roles, timely communication, and coordinated response strategies enhances patient safety, supports accurate diagnosis, and ultimately improves clinical outcomes.