title: Repeatability of refraction and corrected visual acuity in keratoconus. The CLEK Study Group. Collaborative Longitudinal Evaluation of Keratoconus.
abstract: OBJECTIVE
  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 degrees. 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 degrees. 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.
authors.affiliation: University of Missouri-St. Louis School of Optometry, USA.
authors.forename: L J
authors.initials: LJ
authors.lastname: Davis
authors.forename: K B
authors.initials: KB
authors.lastname: Schechtman
authors.forename: C G
authors.initials: CG
authors.lastname: Begley
authors.forename: J A
authors.initials: JA
authors.lastname: Shin
authors.forename: K
authors.initials: K
authors.lastname: Zadnik
mesh_terms: D000328:Adult; D003261:Contact Lenses; D005260:Female; D006801:Humans; D007640:Keratoconus / Q000503:physiopathology* / Q000628:therapy; D008297:Male; D012029:Refraction, Ocular; D015203:Reproducibility of Results; D014792:Visual Acuity