Tear Assessment Tests: Schirmer, TBUT, Phenol Red Thread, and Dry Eye Grading

Why Assess Tears Before Fitting Contact Lenses?

An adequate tear film is essential for successful contact lens wear. Contact lenses depend on the tear film for lubrication, oxygen transmission, debris removal, and surface wetting. Patients with insufficient tear quantity or poor tear quality are at higher risk for discomfort, reduced wearing time, corneal complications, and ultimately contact lens dropout. Performing tear assessment tests before fitting helps identify patients who may need modified approaches or additional management.

Schirmer Test

The Schirmer test measures aqueous tear production by quantifying the amount of tears produced over a timed interval using a standardized filter paper strip.

Schirmer I Test (Without Anesthesia)

The Schirmer I test measures total tearing, which includes both basal (resting) and reflex tear production:

1. Use a standardized 5mm x 35mm Whatman #41 filter paper strip
2. Fold the strip at the notch and hook it over the lower lid margin at the junction of the middle and lateral thirds
3. Ask the patient to close their eyes gently (or look slightly downward with eyes open)
4. Time for exactly 5 minutes
5. Remove the strip and measure the length of wetting from the notch

Interpretation:

• Normal: Greater than 10mm of wetting in 5 minutes
• Borderline: 5 to 10mm
• Abnormal (aqueous deficiency): Less than 5mm

Schirmer II Test (With Anesthesia)

The Schirmer II test isolates basal tear production by eliminating reflex tearing:

1. Instill a topical anesthetic to eliminate the reflex tearing triggered by the paper strip's irritation
2. Wait for the anesthetic to take effect
3. Perform the test as above

Because reflex tearing is eliminated, the Schirmer II results are typically lower than Schirmer I. Values less than 5mm in 5 minutes suggest true aqueous deficiency rather than compensated reflex tearing.

Tear Break-Up Time (TBUT)

TBUT evaluates tear film stability rather than quantity. It measures how long the tear film maintains its uniform coating over the cornea after a blink.

Procedure:

1. Instill a small amount of sodium fluorescein into the lower fornix using a moistened fluorescein strip
2. Ask the patient to blink several times to distribute the dye
3. Ask the patient to blink once more and then hold their eyes open
4. Observe the tear film under cobalt blue illumination at the slit lamp
5. Time from the last blink to the appearance of the first dark spot (dry spot) on the cornea
6. Repeat at least three times and average the results

Interpretation:

• Normal: 10 seconds or more
• Borderline: 5 to 10 seconds
• Abnormal (unstable tear film): Less than 5 seconds

A short TBUT suggests problems with the lipid layer (allowing excessive evaporation) or the mucin layer (poor tear spreading). Unlike the Schirmer test, which measures tear quantity, TBUT assesses tear quality and stability.

Phenol Red Thread (PRT) Test

The Phenol Red Thread test is a less invasive alternative to the Schirmer test. It uses a thin cotton thread impregnated with phenol red, a pH-sensitive dye that changes from yellow to red when it contacts the alkaline tear fluid.

Procedure:

1. Place the thread over the lower lid margin (similar to Schirmer strip placement)
2. Leave in place for only 15 seconds (much shorter than the 5-minute Schirmer test)
3. Remove and measure the length of color change (from yellow to red)

Interpretation:

• Normal: 10mm or more of wetting in 15 seconds
• Abnormal: Less than 10mm

Advantages of the PRT test over Schirmer testing include shorter test time, less patient discomfort, and less reflex tearing stimulation. However, it is less widely validated than the Schirmer test.

Tear Meniscus Height

The tear meniscus height is the strip of tears visible along the lower lid margin, observed at the slit lamp with white light. It provides a quick estimate of tear volume without requiring special strips or dyes.

• Normal: Approximately 0.2 to 0.4mm
• Reduced: Less than 0.2mm suggests decreased tear volume

Dry Eye Grading and Classification

Comprehensive dry eye assessment combines multiple tests and observations into a grading system:

• Symptoms: Patient-reported dryness, burning, grittiness, foreign body sensation, fluctuating vision
• Tear quantity: Schirmer test, PRT test, tear meniscus height
• Tear stability: TBUT
• Ocular surface assessment: Corneal and conjunctival staining with fluorescein and lissamine green
• Meibomian gland evaluation: Lid margin inspection, gland expression

Dry eye is broadly classified into two categories:

• Aqueous deficient: Insufficient tear production (low Schirmer values, reduced meniscus height)
• Evaporative: Adequate production but excessive tear evaporation, usually due to meibomian gland dysfunction (low TBUT with normal Schirmer values)

Many patients have a combination of both types (mixed dry eye).

Key Takeaways

• Schirmer I (no anesthesia) measures total tears; Schirmer II (with anesthesia) measures basal tears
• Schirmer values less than 5mm in 5 minutes suggest aqueous deficiency
• TBUT less than 10 seconds indicates tear film instability
• Phenol Red Thread test is a faster, less invasive alternative to Schirmer (15 seconds vs. 5 minutes)
• Dry eye is classified as aqueous deficient, evaporative, or mixed
• Multiple tests should be used together for accurate dry eye assessment