[Factors influencing the variability of tear film break-up time].

Tear film break-up time was measured on 6 consecutive workdays in 6 subjects without eye disease selected at random. The reasons for the wide range of scatter in the values measured were investigated by variance component estimation. It was found that intraindividual variance is by far the most important factor influencing the overall variance of the measured values. Therefore, caution is called for when judging the stability of the precorneal tear film by the tear film break-up time in healthy individuals.