TBUT and NITBUT (tear break-up time and noninvasive tear break-up time, respectively) were measured on four groups of subjects from different countries (two groups of Caucasians, two groups of Chinese). No significant difference was found in either TBUT or NITBUT values among the four groups of subjects. Gender of the subjects was not a factor affecting the TBUT or NITBUT values. The distributions of both NITBUT and TBUT were non- Gaussian and the median value of either parameter was lower than those previously reported. The NITBUT values were significantly higher than the TBUT values in both Caucasian and Chinese subjects. An investigation into the extent of agreement between the TBUT and NITBUT methods of assessing tear stability indicated that these methods do not agree sufficiently closely (that is, tear stability values measured with the NITBUT technique do not agree with those measured with the TBUT technique); short average tear break-up values are associated with small differences between the measures, whereas long average tear break-up values are associated with large differences between the measures.
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