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Visual acuity repeatability in keratoconus: impact on sample size. Collaborative Longitudinal Evaluation of
M O Gordon1, K B Schechtman, L J Davis
1Washington University Medical School, Department of Ophthalmology & Visual Sciences, St. Louis, Missouri, USA.
Summary
Visual acuity measurements in keratoconus patients are highly repeatable, even with varying vision. Repeatability slightly decreases when different examiners conduct tests, impacting sample size calculations for studies.
Area of Science:
- Ophthalmology
- Optometry
- Vision Science
Background:
- Keratoconus is a progressive eye condition causing irregular cornea shape.
- Accurate visual acuity measurement is crucial for monitoring keratoconus progression and treatment efficacy.
- Understanding measurement repeatability is essential for designing robust clinical studies.
Purpose of the Study:
- To determine the repeatability of visual acuity (VA) measurements in keratoconus patients.
- To assess the impact of VA measurement repeatability on required sample sizes in clinical trials.
Main Methods:
- A subset of 134 keratoconus patients from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study underwent repeat VA testing.
- Visual acuity was measured using high and low contrast Bailey-Lovie charts with habitual and best spectacle/contact lens corrections.
- Testing was conducted at two visits separated by a median of 90 days.
Main Results:
- Mean absolute differences in VA letter counts ranged from 3.24 to 5.48.
- Intraclass correlation coefficients indicated good to excellent repeatability (0.757–0.853).
- Slightly lower repeatability was observed when different examiners performed tests at baseline and repeat visits.
Conclusions:
- Visual acuity measurements in keratoconus patients demonstrate high repeatability.
- Repeatability is adequate for clinical research, though examiner consistency is recommended.
- Repeatability data can inform sample size calculations for keratoconus studies.