*3.3.4.1 Tear film break-up time (TBUT)*

This is the time interval between the last complete blink and the appearance of the first randomly distributed dry spot [46, 47]. It is the most commonly done test for assessing the tear film stability.

It can either be done with or without fluorescein 2% dye. When done with dye it is known as Fluorescein break-up time (FBUT). The dye enhances the visibility of tear film but it also reduces the stability of the tear film and therefore the measurement may not be accurate [48, 49].

Method- Fluorescein 2% is instilled in the eye. It can be instilled in varying volume and concentrations either by impregnated strips or micropipette. A standardized method is to be followed every time and instructions are given to naturally blink thrice than stop blinking until instructed again [50].

A cut-off of < 10 seconds is used to diagnose DED. In patients of Sjogren syndrome, the sensitivity and specificity of the test have been reported to be 72.2% and 61.6%, respectively [7].
