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David J. Russell, Shamis Fallah, Christopher J. Loer, Robert H. Riffenburgh; A Comprehensive Model for Correcting RNFL Readings of Varying Signal Strengths in Cirrus Optical Coherence Tomography. Invest. Ophthalmol. Vis. Sci. 2014;55(11):7297-7302. doi: 10.1167/iovs.14-14993.
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To develop a model for the Cirrus HD-OCT that allows for the comparison of retinal nerve fiber layer (RNFL) thickness measurements with dissimilar signal strengths (SS) and accounts for test–retest variability.
Retinal nerve fiber layers were obtained in normals using the Cirrus optic disc cube 200 × 200 protocol during a single encounter. Five RNFL scans were obtained with a SS of 9 or 10. Diffusion lens filters were used to degrade SS to obtain five scans at each SS group of 7 or 8, 5 or 6, and 3 or 4. The relationship between average RNFL thickness and SS was established, and an equation was developed to allow for adjustment of an RNFL measurement had it been a SS of 7. Intravisit interclass correlation coefficient (ICC) and coefficient of variation (CV) parameter estimates for each SS group were calculated. Repeatability and upper tolerance limit were calculated as 1.96 × √2 × within-subject standard deviation (Sw) and 1.645 × √2 × Sw, respectively.
There was a linear relationship between average RNFL and SS. RNFLadj = RNFL − 1.03*SS + 7.21 allows for the adjustment of RNFL readings to the same SS. Interclass correlation coefficients and CVs were good for all measurements down to SS of 3 or 4. Repeatability and upper tolerance limit were 5.24 and 4.40 μm, respectively.
Our model adjusts RNFL readings based on SS and includes an upper tolerance limit of 5 μm. If validated, this model could improve the detection of real RNFL changes. Further study to validate this model should be performed before widespread use is adopted.
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