title: A comprehensive model for correcting RNFL readings of varying signal strengths in cirrus optical coherence tomography.
abstract: OBJECTIVE
  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.
authors.affiliation: Naval Medical Center, San Diego, San Diego, California, United States.
authors.forename: David J
authors.initials: DJ
authors.lastname: Russell
authors.affiliation: Naval Medical Center, San Diego, San Diego, California, United States.
authors.forename: Shamis
authors.initials: S
authors.lastname: Fallah
authors.affiliation: Naval Medical Center, San Diego, San Diego, California, United States.
authors.forename: Christopher J
authors.initials: CJ
authors.lastname: Loer
authors.affiliation: Naval Medical Center, San Diego, San Diego, California, United States.
authors.forename: Robert H
authors.initials: RH
authors.lastname: Riffenburgh
mesh_terms: D000465:Algorithms; D006801:Humans; D008962:Models, Theoretical; D009412:Nerve Fibers / Q000473:pathology*; D009898:Optic Disk / Q000473:pathology*; D009901:Optic Nerve Diseases / Q000175:diagnosis* / Q000503:physiopathology; D012165:Retinal Ganglion Cells / Q000473:pathology*; D041623:Tomography, Optical Coherence / Q000379:methods*; D014794:Visual Fields
keywords: Cirrus OCT; glaucoma anterior segment; optical coherence tomography; signal strength