The potential influence of Schirmer strip variables on dry eye disease characterisation, and on tear collection and analysis

Purpose: The use of the Schirmer strips (SS) as a tool in the characterisation of dry eye disease, depends upon the quantitative assessment of tear production and constituents. The aim of this study was to ascertain the extent to which the properties of commercially available SS can vary and the way in which this baseline information may relate to their comparability in clinical use. Methods: Five SS were analysed: Clement Clarke®, TearFlo®, Bio Schirmer®, Omni Schirmer® and JingMing®. Various aspects of their physical appearance and physicochemical behaviour were measured, including size, weight, and thickness together with surface morphology (assessed by SEM) and aqueous uptake and release behaviour (including the influence of each strip on protein retention and eluent osmolarity). Results: All physical parameters varied between the strips studied for example the Clement Clark was the largest, thickest, and heaviest strip assessed in this study. SEM images showed that each of the SS had unique surface morphologies. Statistically significant differences among the strips were found for uptake (p = 0.001) and release volume (p = 0.014). Clement Clarke absorbed the highest volume over a fixed time period (23.8 ± 1.6 μl) and Omni the lowest (19.3 ± 0.5 μl). Clement Clarke showing the highest eluent osmolarity value (5.0 ± 0.0 mOsm/L) and TearFlo the lowest (2.8 ± 0.4 mOsm/L). Conclusion: The five strips investigated in this study indicate that there is no standardisation of commercial strips, despite the fact that the need for standardisation was recognised over fifty years ago. This study provides useful baseline information relating to SS comparability in clinical use.