TBUT

Tear break-up time (TBUT) is a clinical measure of tear film stability. It is defined as the interval between a complete blink and the first appearance of a dry spot on the tear film over the cornea. Short TBUT indicates tear film instability, which is commonly associated with dry eye disease.

TBUT is usually measured at the slit lamp after instilling a small amount of fluorescein dye in the conjunctival sac. The patient is asked to blink and then keep the eye open; with a cobalt blue filter, the clinician times from the last blink to the first appearance of fluorescein-dyed tear film breaks on the corneal surface. The measurement is repeated for each eye; the average may be reported. Non-invasive methods exist that estimate TBUT without dye, using cameras or placido rings or interferometry to detect tear film disruption.

Normal TBUT values are typically about 10-15 seconds in healthy individuals, with some sources considering greater than 10 seconds as normal. Values below about 10 seconds suggest tear film instability and may reflect evaporative or aqueous-deficient dry eye.

TBUT can be affected by blinking, fluorescein concentration, and technique; staining can alter tear dynamics; observer variability; the test is influenced by environmental conditions and patient factors; it has limited specificity and may change with contact lens wear, medications, and age.

TBUT is widely used in the assessment of dry eye disease, especially evaporative forms like meibomian gland dysfunction, and to monitor response to therapies such as artificial tears, anti-inflammatory medications, or eyelid hygiene.