In testing for TBUT, sodium fluorescein dye is added to the eye and the tear film is observed under the slit lamp while the patient avoids blinking until tiny dry spots develop. The longer it takes, the more stable the tear film. A short tear break-up time is a sign of a poor tear film. Generally, >10 seconds is thought to be normal, 5 to 10 seconds, marginal, and <5 seconds low (with high likelihood of dry eye symptoms).
An unstable tear film can explain dry eye symptoms in patients who have a normal quantity of tears. Unstable means that the composition of the tears is imbalanced, resulting in tears evaporating too quickly or not adhering properly to the surface of the eye.
Meibomiam gland dysfunction, where not enough lipid is secreted by the meibomian glands to "seal" the aqueous tears and retard evaporation, is a common cause of tear film instability.