Repeatability of Corneal Topography Measurement in Keratoconus with the TMS-1

Purpose. The purpose of this study was to report the test–retest variability of simulated indices derived from the TMS-1 topography instrument (Tomey Technology, Waltham, MA) in keratoconus subjects enrolled in the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study. Methods. Four images were taken at an initial visit and at a repeat visit several weeks later. From these images, 17 indices were simulated from published formulas. Mixed-model analysis was used on test–retest data from the TMS-1 videokeratography instrument during the baseline year. This analysis yields estimates of within- and between-visit variability. Results. Repeatability analysis revealed that within-visit standard errors were 1.0 to 5.9 times greater in keratoconus eyes than in normal controls when two images were analyzed from each visit. These values changed only slightly when more images were used. The ratio of between-visit standard errors of the indices were nearly equally greater than normal controls for (0.9 – 4.6 and 0.9 – 4.3) two images per eye and all images per eye, respectively. Conclusions. These results suggest that the repeatability of simulated indices derived from TMS-1 topography in keratoconus subjects is poorer than in normal controls.