Purpose.: To compare retinal nerve fiber layer (RNFL) measurements obtained by two spectral-domain optical coherence tomographies (SD OCTs; Cirrus-HD OCT and RTVue) and a time domain OCT (TD OCT; Stratus OCT). The diagnostic ability of the three instruments for glaucoma has also been compared. Methods.: One hundred eight open-angle glaucoma patients and 46 normal controls were enrolled. Three OCT scans were repeated to obtain peripapillary RNFL thickness measurements from all participants on the same day. The relationships between RNFL measurements from three OCTs were evaluated with regression analyses. Results.: The thickest RNFL measurements were generally obtained with the RTVue, followed by the Stratus, and finally by the Cirrus OCT (mixed model, p < 0.05). However, the tendency was reversed or no longer present in severe glaucomatous eyes and nasal quadrant maps. Regression analysis between the TD OCT and the two SD OCTs revealed a quadratic relation (linear vs. non-linear, Akaike information criterion, all p < 0.05), whereby RNFL measurements of the TD OCT became exponentially thinner than those by the SD OCTs in thin RNFL thicknesses. Comparing the two SD OCTs, RTVue measured thicker values consistently regardless of the RNFL thickness (pairwise comparisons, p < 0.05). There were no significant differences in the diagnostic ability of all three instruments in average and each quadrants of RNFL thickness (p > 0.05). Conclusions.: Direct comparisons of RNFL thickness measurements among OCT instruments may be misleading as there are considerable differences among devices. The measurement differences between SD OCT and TD OCT differed according to the RNFL thickness and also possibly to severity of the glaucoma. The diagnostic ability of each instrument for glaucoma detection was not significantly different.
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