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J. J. Nichols, K. S. Reuter, P. E. King-Smith; Variability of and Relation Between Interferometric Measures of the Tear Film. Invest. Ophthalmol. Vis. Sci. 2009;50(13):1231.
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Interferometric measures of tear film and lipid layer thickness are a non-invasive, optical means of capturing potentially clinically relevant information about the tear film. The purpose of this work was to examine the variability of the tear film using this method and the relation between the lipid layer and tear film thinning.
Non-contact lens wearers and soft contact lens wearers were examined on two separate occasions (separated by 24 hours) at approximately the same time of day. A previously described interferometric system, based on wavelength-dependent fringes, was used to measure lipid layer, full precorneal or pre-lens tear film thickness (PCTF or PLTF), and thinning of the PCTF and PLTF on each occasion. Differences in each outcome between visits was examined by the Wilcoxon signed ranks test. Intraclass correlation coefficients (ICC) and 95% limits of agreement (LoA) were used to characterize variability of the tear film., and Spearman’s correlation was used to examine the relation between overall average lipid layer thicknesses and tear film thinning.
Fifty-two non-lens wearers (age = 33.7 ± 12.5 year, 61% female) and 47 lens wearers (age = 27.5 ± 7.3 year, 81% female) were enrolled in the study. For non-lens wearers, there was no difference in the average values for lipid layer (p = 0.09), PCTF thickness(p = 0.36), PCTF thinning (p = 0.99) when comparing test sessions. Within-subject reliability was as follows: lipid layer (ICC = 0.84, 95% LoA = -25.27 to 22.25 nm), PCTF thickness (ICC = 0.67, 95% LoA = -2.29 to 2.53 µm), PCTF thinning (ICC = 0.56, 95% LoA = -8.20 to 9.13 µm/min). For contact lens wearers, there was no difference in PLTF thickness (p = 0.52), but there was a difference in lipid thickness (p = 0.02) and PLTF thinning (p = 0.04) when comparing test sessions. Within-subject reliability was as follows: lipid layer (ICC = 0.56, 95% LoA = -17.84 to 27.85 nm), PLTF thickness (ICC = 0.75, 95% LoA = -1.63 to 1.70 µm), and PLTF thinning (ICC = 0.57, 95% LoA = -7.30 to 11.91 µm/min). For both non-lens wearers and lens wearers, there was a strong relation between lipid layer thickness and tear film thinning (r = 0.39 and r = 0.60, respectively; p < 0.007 for both correlations).
The tear film is a dynamic system that shows some variability that should be considered relative to methods used in its analysis. Imaging systems, which could average over larger areas, may help reduce observed variability. For both non-lens and lens wearers, thicker lipid layers are strongly associated with slower rates of tear film thinning.
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