Purpose: To assess the impact of OCT signal strength (SS) and artifact on retinal nerve fiber layer (RNFL) measurement reliability and to understand whether glaucoma severity modifies this relationship. Design: Retrospective, longitudinal cohort study. Participants: Two thousand nine hundred ninety-two OCT scans from 474 eyes of 241 patients with glaucoma or glaucoma suspect status. Methods: We extracted mean RNFL thickness and SS and manually graded scans for artifact. To analyze the effect of SS and artifact on OCT reliability, we (1) created a multilevel linear model using measured RNFL thickness values and demographic and clinical data to estimate the true (predicted) RNFL thickness, (2) calculated model residuals (ΔRNFL) as our reliability measure, and (3) created a second multilevel linear model with splines and interaction terms that modeled overall and quadrant specific reliability (ΔRNFL) as the outcome, using SS and artifact as predictors. Main Outcome Measures: Impact of SS and artifact on ΔRNFL. Results: For SS between 10 and 3, the impact of decreases in SS on OCT reliability is modest (–0.67 to –1.25 ΔRNFL per 1-point decrease in SS; P < 0.05). But at less than 3, changes in SS have a large impact on reliability (–15.70 to –16.34 ΔRNFL per 1-point decrease in SS; P < 0.05). At SS between 10 and 3, decreases in SS tend to have a larger impact on reliability in eyes with severe glaucoma (–1.25 per 1-point decrease in SS; P < 0.05) compared with eyes with mild or moderate glaucoma (–0.67 to –0.75 per 1-point decrease in SS; P < 0.05). The presence of artifact has a significant impact on OCT reliability independent of the effects of SS (–4.76 ΔRNFL; P < 0.05). Artifact affects reliability solely in the quadrant in which it occurs, with artifact in one quadrant showing no impact on ΔRNFL in the opposite quadrant (P > 0.05). Conclusions: Signal strength decreases down to 3 have relatively mild impacts on OCT reliability. At less than 3, the impact of further decreases in SS on reliability are substantial. The effect of SS on reliability is greater in severe glaucoma. Artifacts result in a decrease in reliability independent of the effect of SS. We propose evidence-based guidelines to guide physicians on whether to trust the results of an OCT scan.
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