The assessment of automated measures of hydrogel contact lens refractive index

Purpose: To investigate within- and between-operator reliability and validity of refractive index measures of hydrogel lenses. Methods: Twenty-three lenses of various nominal water contents were examined by two operators on two occasions separated by 1 h. An automated refractometer (CLR 12-70, Index Instruments, Cambridge, UK) was used for refractive index measures. Lenses were presented in a random order to each operator by a third party, and operators were masked to any potential lens identifiers. Intraclass correlation coefficients (ICC), 95% limits of agreement (LoA), and paired t-tests were used to characterize the within- and between-operator reliability and validity of lens refractive index measures. Results: The mean (+/-S.D.) difference between sessions was 0.0001 +/- 0.0011, which did not significantly differ from zero (p = 0.64). Within-operator reliability was excellent (ICC = 0.9990, 95% LoA = -0.0021 to 0.0024). The mean (+/-S.D.) difference between-operators was 0.0002 +/- 0.0010, which was not significantly different from zero (p = 0.54). Between-operator reliability was also excellent (ICC = 0.9984, 95% LoA = -0.0026 to 0.0029). The mean difference between the nominally reported refractive index and our refractive index measures was -0.0009 +/- 0.082, which did not differ statistically from zero (p = 0.52); the 95% LoA for this difference was -0.0152 to 0.0169. Conclusion: There is good reliability within- and between-operators in measuring the refractive index of hydrogel lenses using the CLR 12-70 automated refractometer.