Tear film breakup time (BUT) was measured in 50 normal subjects. The reproducibility of this value, its relationship to age and sex, the effects of lid holding, and the instillation of a local anesthetic were studied. Results have defined a range of normal values and indicate that holding the lids and the instillation of a local anesthetic significantly reduce BUT. No relationship between BUT and age, sex, or corneal sensation was noted. Breakup time is a reproducible phenomenon in an individual eye.
Lemp MA, Hamill JR. Factors Affecting Tear Film Breakup in Normal Eyes. Arch Ophthalmol. 1973;89(2):103-105. doi:10.1001/archopht.1973.01000040105007