Tear Film Break Up Time (TBUT)
TBUT is an indication of tear film stability. The proper method of TBUT testing is using a wet fluorescein-impregnated strip. The dye is distributed by blinking, and the patient is then asked to stare straight ahead without blinking. The tear film is observed under the cobalt blue light of a slit lamp, and the time between the last blink and the appearance of the first dry spot or hole in the tear film is measured and equal to the TBUT.
TBUT has been shown to be decreased in keratoconjunctivitis sicca, mucin deficiency, and meibomian gland disease. Normal subjects show variability in TBUT, although 10 seconds is the typical cutoff between normal and abnormal results and has been found to be relatively specific in screening patients for tear film instability.