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Daniel R. Powell, Jason J. Nichols, Lisa A. Jones-Jordan, Peter E. King-Smith, Kelly K. Nichols; Lipid Layer Thickness Comparison Using Interferometry and a Novel Video Microscopy-Based Method. Invest. Ophthalmol. Vis. Sci. 2011;52(14):3737.
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To determine if an association exists between lipid layer thicknesses (LLT) measured by interferometry and a video microscopy-based method.
Data were collected from 205 confirmed post-menopausal women over the age of 50 years. Wavelength-dependent fringe (WDF) based interferometry followed by lipid layer microscopy was conducted on the right eye of each subject. Spectral reflectance WDF interferometry was used to determine LLT within a 33 µm-diameter area on the central corneal tear film over 20 sec., while LLT for lipid layer microscopy was measured by calculating imaging reflectance (as a function of LLT) within a 200 µm-diameter circular area using a high resolution video microscope over 20 sec. Pearson and Bland-Altman analyses were used to evaluate for correlation and agreement between the LLT from the two systems.
The average LLT was 41.96 ± 23.42 nm and 48.02 ± 13.38 nm for the interferometry and lipid layer microscopy systems, respectively. There appears to be a moderate association in LLT measurements for both methods (r = 0.36, p < 0.001). The mean difference (see figure) between the interferometry and lipid later microscopy LLT measures was 6.06 ± 22.44 nm, which differed significantly from zero (paired t-test = 3.85, p < 0.001). The 95% limits of agreement for this difference were -37.93 to 50.05 nm. The variances for the two measurements were significantly different (F = 21.26, p < 0.001), although there appears to be a greater agreement when the mean LLT measurements of the two methods are below 60 nm.
The video capabilities of the lipid microscope allow for the additional qualitative assessment of the tear film lipid layer. In addition, lipid microscopy may serve as a beneficial tool in the quantitative assessment of LLT, although caution must be taken into account when interpreting thick LLT values above 90 nm (maximum reflectance at this thickness) as these values may be misinterpreted as being less than their actual thickness, which is more accurately derived from the WDF interferometry system.
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