Purpose: To assess the effect of signal strength (SS) on the reproducibility of circumpapillary retinal nerve fiber layer (cpRNFL) thickness measurement (measurement agreement) and its color-coded classification (classification agreement) by Cirrus spectral-domain optical coherence tomography (OCT). Methods: Seven hundred and eighty-nine participants, consecutively enrolled from December 2009 to August 2010, underwent two repeated Cirrus OCT scans to measure cpRNFL thickness (optic disc cube 200 × 200). Intraclass correlation (ICC) and linear-weighted kappa coefficient (κ) were calculated as indicators for cpRNFL measurement and classification agreement. The difference in cpRNFL thickness measurements between repeated OCT scans (inter-scan measurement difference) was correlated with mean SS, SS change or difference between repeated OCT scans (inter-scan SS difference), and average cpRNFL thickness. Results: Mean SS was negatively correlated with inter-scan SS difference and was positively correlated with the inter-scan measurement difference. Repeated scans with SS change (inter-scan SS difference = 1 or 2) showed larger variability than scans without SS change (inter-scan SS difference = 0) in total average cpRNFL and all quadrant maps except for the nasal quadrant. Multivariate analysis revealed that inter-scan measurement differences increased in subjects with thinner cpRNFL thicknesses and in scans with lower mean SS and higher inter-scan SS differences. Measurement agreement (ICC) in groups with larger inter-scan SS differences (=2) was lower than those with smaller inter-scan SS differences (=0 or 1) in their average cpRNFL and inferior quadrant maps. Classification agreement (κ) in groups with larger inter-scan SS differences (=2) was lower than those with smaller inter-scan SS differences (=0 or 1) in their average cpRNFL and quadrant maps (superior and inferior quadrant) although statistically not significant. Conclusions: Special attention should be paid when comparing the cpRNFL thickness measurement and color-coded classification of 2 or more Cirrus OCT scans as the agreements may be susceptible to SS differences.
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