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W.J. Benjamin, The Dk Reference Study Group; Revised Oxygen Permeability (Dk) Of Reference Materials . Invest. Ophthalmol. Vis. Sci. 2006;47(13):97.
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© ARVO (1962-2015); The Authors (2016-present)
The Permeability Reference Material Repository was established in the late 1990’s and has since supplied reference buttons from single lots of 7 rigid gas–permeable (RGP) materials to most of the known measurers of oxygen permeability (Dk) for contact lens materials. The standard Dk values for 6 of the materials were tentatively established using the initial 4 sets of Dk values (2 polarographic and 2 coulometric) by linear calibration to those determined by Dr. Irving Fatt. The 7th material was acquired later and its standard Dk was estimated by extrapolation of the linear relationship between the measured and standard Dk values of the other materials. The early reference Dk values are updated here using data accumulated over the last 8 years.
The mean Dk values (n = 4) previously established were 10.0, 26.0, 51.2, 62.4, 92.9, 133.6, and 161.1 Fatt Dk units, respectively, for silafocon A, paflufocon C, itafluorofocon A, tolofocon A, siflufocon B, melafocon A, and tisilfocon A. Fifteen (15) sets of Dk values have by now been obtained for the repository materials: 9 polarographic, 4 coulometric, and 2 gas–to–gas. All polarographic Dk values were corrected for the boundary layer and edge effects according to the American National Standards Institute (ANSI) Z80.20 standard. The coulometric and gas–to–gas Dk values were measured for these rigid materials in a manner that circumvented these effects. Each measured Dk value of a set was calibrated with a linear regression involving the measured and standard Dk values of the other reference materials. The mean Dk values (n = 15) of the reference materials were then substituted for their initial standard Dk values and a 2nd iteration of the calibration was performed. Then a 3rd iteration was performed.
There was less than a 2% difference between the calibrated Dk values of the 2nd and 3rd iterations, the mean difference being 0.54%. The revised reference Dk values were 8.8, 26.8, 51.8, 61.8, 101.0, 130.9, and 154.3 Fatt Dk units. Linear regressions between the 15 sets of measured Dk values and the revised references resulted in coefficients of determination ranging from 0.933 to 0.998. Ten (10) of the 15 coefficients were above 0.990 and 12 were above 0.980.
These results show that Dk values from 10 different operators at 8 different sites using the 3 primary fundamental methods of Dk assessment (polarographic, coulometric, and gas–to–gas) were highly correlated with the revised reference values in a linear fashion. A logical next step in the confirmation of this method of calibration would be to analyze the reproducibility of these data sets before and after calibration.
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