The oily layer of the tear film protects the aqueous layer from evaporating, and lubricates the mucosal tissue surrounding the eye. When the oil breaks up, the eye senses dryness, and we blink automatically. A healthy oily layer should not break up for at least 10 seconds. Any less than that indicates a problem with meibomian gland secretions.
To test TBUT, a drop of fluorescein dye is placed in the eye, the patient blinks a few times to spread the dye across the eye’s surface, and the doctor counts the seconds before the tear film breaks up.
Any value above 10 seconds is considered normal. A value below 10 indicates that the lipid layer of the tear film is compromised.
Reference
Tear film breakup time (B.U.T.) in sub clinical dry eyes
Kulshrestha OP, Talwar P, Mathur M.
Indian Journal of Ophthalmology
1984 [cited 2015 Oct 14];32:454-5
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