A comprehensive model for correcting RNFL readings of varying signal strengths in cirrus optical coherence tomography.

PURPOSE 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. METHODS 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. RESULTS 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. CONCLUSIONS 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.