Repeatability of Epithelium Thickness Measured by an AS-OCT in Different Grades of Keratoconus and Compared to AS-OCT/Placido Topography

PURPOSE: To compare agreement of corneal epithelium thickness (ET) between AS-OCT system (RTVue, Optovue) and AS-OCT/Placido topographer (MS-39, CSO) in eyes with different stages of keratoconus (KC), and to assess the repeatability of RTVue AS-OCT. DESIGN: Prospective reliability analysis. METHODS: KC eyes were classified into forme fruste KC (FFKC), mild, moderate, and severe KC. Agreement was evaluated with Bland–Altman plots and 95% limits of agreement (LoA). The repeatability of RTVue was assessed via within-subject standard deviation (S_w), test–retest variability (TRT), coefficient of variation (CoV), and intraclass correlation coefficient (ICC). RESULTS: A total of 119 KC eyes were enrolled, with 21 being FFKC, 26 mild, 39 moderate, and 34 severe. The 95% LoA ranged between −5.9 and 4.8 µm for center epithelium thickness (CET), between −5.7 and 8.2 µm for thinnest epithelium thickness (TET). At 1-mm measuring points, the 95% LoA of superior, inferior, nasal, and temporal were −4.2 to 4.7 µm, −5.2 to 6.0 µm, −7.9 to 10.2 µm, and −11.2 to 6.0 µm. At 3-mm measuring points, the corresponding values were −2.8 to 9.3 µm, −2.0 to 13.0 µm, −4.6 to 9.6 µm, and −6.3 to 9.7 µm, indicating that the 2 instruments were not interchangeable without adjustment. Despite that the repeatability of RTVue measurements in KC patients were acceptable, repeatability decreased gradually with the peripheralization of the measurement points. CONCLUSIONS: The 2 OCT-based devices, RTVue and MS-39, do not provide interchangeable measurements of epithelium thickness in KC patients. Repeatability decreases in cases of more severe KC, emphasizing the importance of grading before clinical examination to avoid diagnostic errors.