Validation of an Automated Early Treatment Diabetic Retinopathy Study Low-contrast Letter Acuity Test

Abstract

Significance: Automated low-contrast letter acuity (LCLA) has several advantages: consistent luminance, reduced chance of individuals memorizing test letters, and convenient and accurate visual acuity reporting functions. Although automated LCLA might report slightly worse acuity than Sloan LCLA chart, considering its advantages, it may be a viable alternative to Sloan LCLA chart in clinical practice and research.

Purpose: The purpose of this study was to determine the repeatability of an automated LCLA measurement and its agreement with the Sloan LCLA chart test in normal participants and reduced-vision participants.

Methods: Adult participants (n = 49) were measured with both automated Early Treatment Diabetic Retinopathy Study and Sloan LCLA tests, including normal and reduced-vision groups. Low-contrast letter acuity at two contrast levels (2.5 and 10%) was measured at 3 m in a random sequence with both LCLA tests. To test repeatability, participants were retested 1 week later. Repeatability of the two tests between two visits and agreement between automated and Sloan LCLA tests were evaluated using 95% limits of agreement.

Results: In terms of the 95% limits of agreement, the repeatability of both tests was as follows: automated LCLA at 2.5%, ±0.26; automated LCLA at 10%, ±0.22; Sloan LCLA at 2.5%, ±0.23, and Sloan LCLA at 10%, ±0.16. The agreement of the two tests was as follows: ±0.19 at 2.5% and ±0.24 at 10%. The automated LCLA at 2.5 and 10% levels was generally reported one-half to one logMAR line lower than Sloan LCLA (mean differences, -0.04 at 2.5% and -0.13 at 10%; paired t test, P < .05).

Conclusions: The automated LCLA test shows fairly good test-retest repeatability at both 2.5 and 10% contrast levels. The agreement between the automated and the Sloan low-contrast letter acuity tests was comparable with test-retest agreement. Although the automated LCLA test reports slightly worse acuity than the Sloan LCLA test, it could be an appropriate alternative to the Sloan LCLA test.

Figure 1.

Automated ETDRS low contrast letter acuity measurement viewed by the participants (A) Phase I; (B) Phase II with a blue dot) and the examiner (C) Phase I: (D) Phase II).

Figure 2.

Repeatability of the Sloan low-contrast letter acuity (Sloan-LCLA) test and the automated low-contrast letter acuity (automated-LCLA) test is shown in Bland-Altman plots at 2.5% and 10% contrast levels. The LCLA difference between the second and the first administration (second minus first) of each test is plotted against the mean of two tests. (A) Automated-LCLA at 2.5%. (B) Automated-LCLA at 10%. (C) Sloan-LCLA at 2.5%. (D) Sloan-LCLA at 10%. From the top to the middle and the bottom, three lines show the upper 95% LoA, bias, the lower 95% LoA, respectively.

Figure 3.

Bland-Altman plots to demonstrate agreement between Sloan low-contrast letter acuity (Sloan-LCLA) and the automated low-contrast letter acuity (automated-LCLA) at 2.5% and 10% levels (A & B). The difference between the average scores (i.e. Sloan-LCLA minus automated-LCLA) for the two tests is plotted against the mean of two tests. From the top to the middle and the bottom, three lines show the upper 95% LoA, the mean difference, the lower 95% LoA, respectively.