Repeatability of Modified Schirmer Strip Wetted Lengths and Basal Tear Production Rates

Abstract

Purpose : Li et al. (2018 OVS) introduced a modified Schirmer Tear Test (STT) to eliminate evaporation on wetting length and to calculate basal tear production rate (BTPR). We assess the repeatability of the modified STT and the estimated BTPR.

Methods : 17 subjects underwent two sheathed and two unsheathed STTs each bilaterally on four separate visits. Two drops of topical anesthetic were administered, wetted lengths were measured every 30 s for 5 min, and BTPRs were determined following Li et al. Limits of Agreement (LoA) and the Coefficient of Repeatability (CR) assessed repeatability of the sheathed and unsheathed STT strip wetted lengths at 5 min post-insertion, and of BTPRs. Sample size estimates for future group comparisons were made, assuming 95% confidence and 80% power, using estimates of the between-subject variance from study data and from the literature.

Results : For the unsheathed STT, the mean (SD) difference in wetted lengths between visits was 0.74 (5.05) mm, LoA were [-9.17, 10.64] and CR was 9.17 mm; for the sheathed SST, the mean (SD) inter-visit difference was 0.16 (5.94) mm, LoA were [-11.49, 11.8] and CR was 10.53 mm. The mean (SD) difference between repeated visits for BTPR was approximately 0.0 µL/min, LoA were [-1.82, 1.82], and CR was 1.91 µL/min. Sample size estimates show that 126 subjects are sufficient to detect a difference in mean BTPR between two groups as small as 10% under the largest variance estimate. 8 of 48 sheathed STTs had non-linear wetting-length change rate (WLCR), reducing to 0 mm/min during the final 90 s of the test. 20 of 44 unsheathed STTs showed similar behavior. Subjects with non-linear WLCR from the sheathed STT all had wetted lengths <10 mm, a widely accepted threshold for aqueous-deficient dry eye; with the traditional STT, subjects had wetted lengths <19 mm.

Conclusions : The repeatability of sheathed and unsheathed STTs were comparable. The repeatability of both wetted length and BTPR are inadequate for measuring within-subject changes, but are sufficient for group studies with moderate sample sizes. Non-linear WLCR with the sheathed STT indicates time-dependent BTPR, whereas non-linear WLCR with the unsheathed STT can be explained by evaporation and possibly varying BTPR. The modified STT is suggested to eliminate environmental influences and permit estimation of the BTPR.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.