The phenol red thread (PRT) test and tear stabilityof Hong Kong-Chinese

Using a non-invasive tear break-up time (NITBUT) technique, we measured the tear stability of 22 asymptomatic Hong Kong (HK)-Chinese before and after performance of the phenol red thread (PRT) test. One eye was used as the test eye while the fellow eye acted as control. The effect of performing the PRT test before NITBUT measurements on NITBUT values was analysed by comparing the pre- and post PRT test NITBUT values of the test eye, and by comparing the post-PRT test NITBUT values of the test eye with the NITBUT values of the control eye. The median pre- and post-PRT test NITBUT values for the test eye were 129s and 10.4s, respectively; for the control eye, the NITBUT values were 17 7s and 14.7s, respectively. The mean±SD PRT value was 19.6±6.8mm/15s. PostPRT test NITBUT values may be longer or shorter than prePRT test NITBUT values. Similar findings were observed for the control eye between the first and second measurement. Performance of the PRT test before NITBUT measurement does not affect the NITBUT values of asymptomatic HK-Chinese. The performance of the PRT test in one eye also does not affect the NITBUT value of the fellow eye. The same procedures were repeated on 14 subjects using the fluorescein-instillation tear break-up time (TBUT) test instead of the NITBUT test. Performing the PRT test did not affect the TBUT values. The median pre- and post-PRT test TBUT values for the test eye were 3.8s and 4.2s, respectively,- for the control eye, the TBUT values were 5.4s and 4.1s, respectively. The mean±SD PRT value was 19.0±5.Omm/15s. The performance of the PRT test before NITBUT measurements does not appear to affect the locations of tear defects in the NITBUT test; the location of tear defects is not random but is most frequent in the inferior periphery of the cornea.