The phenol red thread tear test: a cross-cultural study

PURPOSE: To examine the results of the phenol red thread tear test in a cross-cultural comparison. METHODS: Two groups of 500 controlled normal subjects who do not wear contact lenses from the United States and Japan were investigated. RESULTS: The mean wet length of the thread for the United States was 23.9 mm (SD 9.5 mm). The mean for Japan was 18.8 mm (SD 8.6 mm). There was a significant difference between the two countries (P < 0.05). Males subjects had significantly longer wet lengths than females for both countries (P < 0.05). There was a moderate correlation between right and left eye results for both countries. CONCLUSIONS: The phenol red thread tear test was found to be easy to administer. Results were in line with current knowledge and theories of the lacrimal system. Results also indicated that this test may disclose subtle differences not previously found with other tear tests.