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J Peter Gierow, Sara Andersson; Does the Phenol Red Thread test and the Schirmer test measure the same thing?. Invest. Ophthalmol. Vis. Sci. 2013;54(15):4334.
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The Schirmer test has for many years been the standard when it comes to measuring the tear production/flow. However, since it is rather time consuming (5 min) and also causes discomfort for the patient it has more recently been substituted in many cases with the Phenol Red Thread test (PRT), which is considerably faster (15 s) and offers less discomfort. The present study is aimed at comparing the results of the two tests, in an effort to verify that they are measuring the same thing, something that has recently been debated.
33 patients (6 males, 27 females; Age 30+14 years) were enrolled in the study. Both eyes were tested with the less invasive PRT first and the wetting of the thread was recorded at 15, 30, 45, 60 and 90 s. The patient was allowed to recover for 15 min and then subjected to the Schirmer test (un-esthesized) and readings were collected at the times indicated above, and at 2, 3, 4 and 5 min.
The Schirmer test reached a plateau at about 2 min, whereas only a somewhat decreased rate was observed at 90 s. When comparing the two recommended measuring times (5 min vs 15 s), no significant correlation was observed, although a trend towards a negative correlation was notable. A significant positive correlation was observed when comparing the two end-points (5 min and 90 s; p<0.05; r=0.41).
Our results indicate that the two methods do not measure the same thing when the two recommended observation times were used (5 min vs 15 s). The 15 s collection time might partly reflect the tear volume present at the meniscus when the PRT test is used, but further studies are required to verify this.
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