Repeatability of refraction and corrected visual acuity in keratoconus

Purpose, The purpose of the test-retest phase of the Collaborative Longitud inal Evaluation of Keratoconus (CLEK) Study was to determine the repeatabil ity of the various parts of the CLEK Study protocol. This paper presents th e test-retest parameters of the refraction protocol. Methods. We examined 1 38 CLEK Study-eligible patients on two occasions (median, 90 days; range, 2 2 to 268 days). All patients underwent subjective refraction on two occasio ns, and contact lens over-refractions were performed either over the patien t'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 we arers. Results. Mean interoccasion differences a SD were -0.32 +/- 2.91 D a nd -0.17 +/- 1.39 D for subjective refraction sphere and cylinder power, re spectively, and the mean absolute difference for subjective refraction cyli nder axis was 18.1 +/- 20.2 degrees. The mean interoccasion difference a 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 cylind er power, respectively, and the mean absolute difference for contact lens o ver-refraction cylinder axis was 11.6 +/- 9.9 degrees. The mean interoccasi on difference +/- SD for visual acuity with contact lens over-refraction wa s 0.50 +/- 5.2 letters correct and 0.71 +/- 6.9 letters correct for high- a nd low-contrast visual acuity, respectively. Conclusions. The repeatability of subjective refraction in keratoconus patients is good but somewhat lowe r than that found in nondiseased eyes. Only 36% of our repeat measures of s phere power from subjective refraction fell within 0.50 D of each other, co mpared with more than 90% in studies of normal eyes.