Comparison of Corneal Epithelial Thickness Measurement Between Fourier-Domain OCT and Very High-Frequency Digital Ultrasound

PURPOSE: To compare measurements of corneal epithelial thickness using optical coherence tomography (OCT) and very high-frequency digital ultrasound (VHFDU). METHODS: Retrospective analysis of 189 virgin corneas and 175 post-laser refractive surgery (LRS) corneas that had corneal epithelial thickness measurement with RTVue Fourier-domain OCT (Optovue, Inc., Fremont, CA) (tear film included) and Artemis VHFDU (ArcScan Inc., Morrison, CO) (tear film excluded). Averages were calculated for the central 2-mm diameter zone and for two further concentric annuli of 1.5- and 0.5-mm width, each divided into eight sectors. Agreement was analyzed by mean difference (OCT - VHFDU), 95% limits of agreement (LoA) (1.96 standard deviation of the difference), and Bland-Altman analysis. RESULTS: In virgin epithelium, mean central thickness was 53.4 +/- 3.20 mu m (range: 46 to 62 mu m) with OCT and 54.1 +/- 2.96 mu m (range: 48 to 61 mu m) with VHFDU; OCT measured thinnest in 70% with a mean difference of -0.71 mu m (95% LoA of +/- 3.94 mu m, P<.001). In post-LRS epithelium, mean central thickness was 57.9 +/- 6.08 mu m (range: 42 to 77 mu m) with OCT and 60.5 +/- 6.47 mu m (range: 42 to 79 mu m) with VHFDU; OCT measured thinnest in 88%, with a mean difference of -2.48 mu m (95% LoA of +/- 5.33 mu m, P<.001). A larger difference between methods was more common with thicker epithelium. CONCLUSIONS: Corneal epithelial thickness measurements using OCT were found to be slightly thinner than for VHFDU. In contrast to VHFDU, OCT measurement includes the tear film, so the true difference is probably approximately 4 mu m more than reported. The difference was greatest inferiorly and higher for post-LRS eyes and in thicker epithelium.