TBUT is a diagnostic test that assesses tear film stability by measuring the time elapsed between the last blink and the appearance of the first tear film break on the corneal surface. The test is performed after applying a drop of sodium fluorescein to the conjunctival sac, which colours the tear film yellow-green and allows it to be visualised under the cobalt-blue light of a slit lamp. The patient is asked to blink several times to distribute the dye evenly and then to refrain from blinking while the examiner observes the surface of the cornea, measuring the time until the appearance of dark spots or lines indicating a tear film break. The correct TBUT value is at least 10 seconds, with values below 10 seconds indicating tear film instability characteristic of the dry eye disease, and results of less than 5 seconds suggest significant impairment.
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