|Title||Which criterion for determining a line visual acuity is the best measure of true visual acuity?|
|Author, Co-Author||Alice Wong, Jason Ng|
Purpose: In clinical and vision screening settings, it is common to use the criterion of 3 or more letters correct on a line (out of 5) to measure a patient’s visual acuity. Although this method is common, evidence in the literature is lacking to support this practice. In this study, the 3 or more letters correct on a line criterion was compared to letter-by-letter acuity, 4 or more and 5 or more correct on a line visual acuity.
Methods: One eye each of 38 subjects (23 females, mean/SD age = 45.4/14.0 yrs.) were tested monocularly with a high-contrast ETDRS visual acuity chart at a distance of 3 meters. Subjects read down the chart until they missed 5 consecutive letters. Four logMAR visual acuity results were then obtained as letter-by-letter acuity and 3 line acuities (3 or more, 4 or more, or 5 correct on a line). Mean visual acuity differences from the 4 criteria were analyzed using repeated measures ANOVA with post-hoc Tukey testing.
Results: The mean/SD visual acuities were -0.09/0.08, -0.11/0.09, -0.06/0.08, and -0.03/0.10 for letter-by-letter acuity, 3 or more correct line acuity, 4 or more correct line acuity, and 5 correct on a line acuity, respectively. The different acuity criteria produced statistically significant differences [F(3,151) = 31.13, p < 0.001]. All criteria produced significantly different mean logMAR visual acuities (p < 0.05) except for letter-by-letter visual acuity and 3 or more line acuity (p > 0.05).
Conclusions: The common practice of using a criterion of 3 or more letters correct on a line to determine a line visual acuity is not significantly different from letter-by-letter acuity. However, 4 or more line acuity and 5 or more line acuity result in a significantly worse acuity estimation. Thus, the use of 3 or more letters correct on a line to assign a line visual acuity is appropriate and provides the best estimate of true visual acuity.
|Affiliation of Co-Authors||Marshall B. Ketchum University|