Repeatability of epithelium thickness measured by an AS-OCT in different grades keratoconus and compared to AS-OCT/Placido topography

Purpose To compare agreement of corneal epithelium thickness (ET) between AS-OCT system (RTVue, Optovue, Fremont, USA) and AS-OCT/Placido topographer (MS-39, CSO, Florence, Italy) in different stages keratoconus (KC) eyes, 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 (Sw), test-retest variability (TRT), coefficient of variation (CoV), and intraclass correlation coefficient (ICC). Results Totally, 119 KC eyes were enrolled, with 21 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 1mm 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, -11.2 to 6.0 μm. At 3mm measuring points, the corresponding values were -2.8 to 9.3 μm, -2.0 to 13.0 μm, -4.6 to 9.6 μm, -6.3 to 9.7 μm, indicating the two instruments weren't interchangeable without adjustment. Despite the repeatability of RTVue in KC patients were acceptable, repeatability decreased gradually with the peripheralization of the measurement points. Conclusions The two OCT-based devices, RTVue and MS-39, don't provide interchangeable measurements of ET in KC patients. Repeatability decreases in severer KC, emphasizing the importance of grading before clinical examination to avoid diagnostic errors.