How Sensitive to Clinical Change Are ETDRS logMAR Visual Acuity Measurements?

Abstract

Abstract: : Purpose: To determine the sensitivity to change, and specificity achieved when published test-retest variability (TRV) data are used to determine whether measured changes in ETDRS logMAR visual acuity reflect true clinical change, or are attributable to measurement error alone. Methods: Various degrees of acuity change were simulated in a group of normal subjects by adjusting test difficulty through manipulation of viewing distance. Sensitivity to simulated change and specificity were determined using ‘change criteria' derived from published Bland-Altman 95% ranges for TRV. Results: The relationship between viewing distance and measured acuity was as predicted theoretically. Simulated acuity change of 0.2 logMAR (two lines of letters) or greater can be reliably distinguished from no change (sensitivity>95% & specificity>95%) with the ETDRS chart, but change of 0.1 logMAR (one line of letters) or less cannot. Conclusions: The use of 95% ranges for TRV to establish the smallest measured visual acuity change which can be reliably detected ensures a high specificity but does not take account of sensitivity. Use of change criteria derived from published 95% ranges will result in a sensitivity of approximately 50% (assuming identical levels of TRV). Sensitivity may be improved by using a change criterion which is smaller than the minimum change sought, providing the change criterion is still at least as large as the 95% range for TRV to such that specificity is maintained. Reducing TRV will allow smaller changes in acuity to be reliably detected.

Keywords: visual acuity • clinical research methodology