Standardising the Schirmer Test by Enclosing the Strip in a Waterproof Sheath

Abstract

PurposeHolly et al. (1984) showed that the wetting length of the Schirmer strip is influenced by evaporation and may be affected by ambient humidity. This could explain poor reproducibility in different environmental conditions. We therefore studied how preventing evaporation affected wetting length in different conditions of relative humidity.MethodsA closed‐eye Schirmer test was performed bilaterally in 8 subjects with normal eyes (3 male; mean age 35.5 years SD ± 25), in a Weiss‐Gallenkamp controlled environmental chamber, at a temperature of 23°C and with a laminar airflow of 0.08 m/s. Tests were conducted at relative humidities (RH) of 25%, 15% and 5%, with a 10 min break between each test. Eyes received either a standard Schirmer paper (Whatman No. 41 – Biotech®), or a Schirmer paper clad in a waterproof sheath, on a random basis. Sheaths were constructed from a double layer of laminating plastic (Fellowes® ImageLast – outer layer: polyethylene terephthalate; inner layer: ethylene‐vinyl acetate), heat‐sealed to form a lightweight pouch. Papers were inserted, first on the right eye and then on the left. Wetting length was measured at 5 minutes. If full wetting occurred before this time, the time was noted.ResultsThe wetting length was greater on the sheathed side in 23/24 tests. At 25% RH the mean difference in wetting length was 39.3% SD ± 14.7 (n = 8), at 15% RH the mean difference was 44.3% SD ± 17.8 (n = 8) and at 5% RH the mean difference was 56.3% SD ± 17.7 (n = 7). However, no significant difference was found in % means at different RH, P (0.287) > α ± (0.05) using a repeated measures ANOVA test.ConclusionsSheathing of the Schirmer strip frees the test from a dependence on ambient conditions that can vary seasonally, geographically and locally from clinic to clinic. Such standardisation would improve the performance and utility of the test.