Line-by-line visual acuity scoring equivalence with letter-by-letter visual acuity scoring.

Clinical relevance: A scoring criterion of three-or-more letters correct on a line results in the most equivalent visual acuity score to letter-by-letter scoring.Background: Using the criterion of three-or-more letters correct on a line of five letters to measure a line visual acuity is common. In this study, different line acuity criteria are compared to letter-by-letter visual acuity scoring to determine which criterion is the most accurate as well as least variable.Methods: One eye each of 32 subjects was tested with high-contrast visual acuity charts at 4.88 m. Subjects had 16 acuities measured: 8 under normal conditions and 8 under a + 1.50D blur condition. For each set of 8, logMAR visual acuity results were obtained by retrospectively applying each of four scoring criteria twice: letter-by-letter acuity and three line acuities (three-or-more, four-or-more, or five correct on a line). Differences in means were analysed using repeated measures ANOVA with post-hoc Tukey testing. Test-retest variability was analysed via Bland-Altman analysisResults: The mean visual acuities (in logMAR) were -0.10, -0.11, -0.07, and -0.01 for letter-by-letter, three-or-more correct, four-or-more correct, and all five correct, respectively. With blur, the mean visual acuities were 0.41, 0.39, 0.45, and 0.53 for the respective criteria. Under both normal and blur conditions, the different acuity scoring criteria resulted in significant differences (p < 0.001), except for the letter-by-letter and three-or-more criteria (p ≥ 0.18). Whereas the criteria resulted in similar test-retest variability under blur, the line acuity using a criterion of three-or-more letters correct resulted in the lowest test-retest variability under best-corrected conditions.Conclusion: A line visual acuity scoring criterion of three-or-more letters correct on a line, whether with good visual acuity or poorer visual acuity due to defocus, provides the most similar acuity and test-retest variability compared to letter-by-letter visual acuity.