Repeatability of Refraction and Corrected Visual Acuity in Keratoconus

Purpose. The purpose of the test-retest phase of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study was to determine the repeatability of the various parts of the CLEK Study protocol. This paper presents the test-retest parameters of the refraction protocol. Methods. We examined 138 CLEK Study-eligible patients on two occasions (median, 90 days; range, 22 to 268 days). All patients underwent subjective refraction on two occasions, and contact lens over-refractions were performed either over the patient's habitual rigid contact lenses or over a trial rigid contact lens equal in base curve to the steep keratometric reading in nonrigid contact lens wearers. Results. Mean interoccasion differences ± SD were -0.32 ± 2.91 D and -0.17 ± 1.39 D for subjective refraction sphere and cylinder power, respectively, and the mean absolute difference for subjective refraction cylinder axis was 18.1 ± 20.2°. The mean interoccasion difference ± SD for high-contrast visual acuity with subjective refraction was 0.38 ± 10.9 letters correct. Mean interoccasion differences ± SD were -0.11 ± 0.81 D and 0.02 ± 0.67 D for contact lens over-refraction sphere and cylinder power, respectively, and the mean absolute difference for contact lens over-refraction cylinder axis was 11.6 ± 9.9°. The mean interoccasion difference ± SD for visual acuity with contact lens over-refraction was 0.50 ± 5.2 letters correct and 0.71 ± 6.9 letters correct for high- and low-contrast visual acuity, respectively. Conclusions. The repeatability of subjective refraction in keratoconus patients is good but somewhat lower than that found in nondiseased eyes. Only 36% of our repeat measures of sphere power from subjective refraction fell within 0.50 D of each other, compared with more than 90% in studies of normal eyes.