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    Tear volume – a neglected issue?

    We report a new tool for measuring tear volume, which is essential for selecting the appropriate treatment for a patient with symptoms of dry eye.

     

    The Lacrymeter

    The PRT test, which was introduced in 1982,14 was developed to overcome many of the disadvantages of the Schirmer test described in the previous section, including high variability, poor reproducibility and low sensitivity for detecting dry eyes.14 Phenol red is pH sensitive and changes from yellow to red when wetted by tears.

    The Lacrymeter is an updated version of the PRT: it is a soft, gentle, minimally invasive product in the form of a thread that soaks up the tear fluid present at the ocular surface over a period of seconds. The colour of the thread changes from yellow to red in response to the tears, and the length of thread that has changed colour correlates with tear volume and flow.

    Methodologically, the lacrymeter is similar to the Schirmer test, although there are some potential advantages that include the fact that there is little to no sensation from the lacrymeter, thus less potential for reflex. Furthermore, the test time is only 30 s per eye (the Schirmer test requires 5 minutes), the eyes remain open and are free to blink and no anaesthetic is required.14–19

    Despite these advantages, a previous test that resembles the lacrymeter has rarely been used in clinical practice or clinical development. Two possible reasons for this are that the threads can be difficult to handle because of their light and flexible nature and that they are only manufactured in Japan, making their supply costly and often requiring special ordering. However, an Israeli company (Fepasaet Group Ltd) has now developed the lacrymeter, which is more accessible and cheaper than other similar devices.

    The lacrymeter test does not require an anaesthetic. The patient must not instill drops 1 hour before the examination; however, contact lenses can be worn during the test.

    Although the PRT and lacrymeter tests are not standard clinical tests, they have the potential to be embraced as such because of their many advantages over the traditional Schirmer I test. Several studies have found the PRT test to be more repeatable than the Schirmer test (with and without anaesthestic) as well as more reliable in diagnosing dry eye.14,19,20 Chiang et al.20 compared 66 normal eyes and 14 dry eyes using both Schirmer I and PRT, performed on successive days in 28 eyes. Comparing normal with dry eyes, the following data were reported: PRT = 20.3 ± 8.7 mm vs 8.1 ± 8.0 mm (P < 0.005); Schirmer = 10.0 ± 7.9 mm vs 14.6 ± 9.8 mm (P = 0.33). Based on these data, the authors concluded that the likelihood of a false positive was 3% using PRT and 18% using Schirmer. An estimate of the reproducibility of the measurements was achieved through comparison of data collected on two successive days. The Pearson coefficient was 0.89 for PRT and only 0.39 for Schirmer. Patel et al.18 concluded that the PRT test could accurately differentiate aqueous-deficient dry eye subjects from non-dry-eye subjects although, in his study, the thread was left in place for 120 s. In the same study, it was concluded that the PRT test could not differentiate between dry eye and non-dry eye if aqueous-deficient and lipid-deficient dry-eyed individuals were combined.

    From a limited number of studies, it generally appears that the PRT outperforms the Schirmer test in the areas of reproducibility and reliability. Global data interpretation, however, must proceed with caution, as several of the studies addressing repeatability were performed on non-dry-eyed subjects, thus calling into question the utility of the findings with respect to use in a dry-eyed population. Nichols et al.21 analysed reliability and correlation of clinical measurements of dry eye and found that positive correlations do exist between (a) Schirmer and fluorescein staining, (b) PRT and both fluorescein and rose bengal staining and (c) Schirmer and PRT.

    Claes Feinbaum
    Claes Feinbaum is Professor Emeritus Optometry at the University of Rostock, Germany. He is an Optometric Consultant for Specspavers ...

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