The effect of optical defocus on the test–retest variability of visual acuity measurements

Abstract

Abstract: : Purpose: To determine the effect of optical defocus upon the test–retest variability (TRV) of visual acuity measurements in normal subjects. Methods: Normal subjects underwent repeated visual acuity measurement with optical defocus of zero, 0.50, and 1.00 dioptres (D). All measurements were taken using the ETDRS version of the Bailey–Lovie logMAR chart. TRV was quantified in terms of its 95% range, both empirically and using the approach of Bland and Altman. Results: Using the Bland and Altman approach, the estimated 95% TRV ranges were ±0.11 logMAR for zero defocus, ±0.18 logMAR for 0.50D defocus, and ±0.25 for 1.00D defocus. Conclusions: Optical defocus has a considerable effect upon the TRV of visual acuity measurements. These findings have important implications for both clinical practice and clinical research. Uncorrected refractive errors as small as 0.50D may compromise the detection of visual change in individuals, and contribute to unnecessarily large sample sizes in clinical trials which employ visual acuity as a primary outcome measure.

Keywords: visual acuity • clinical research methodology