Purpose: The log MAR visual acuity (VA) chart developed for use in the Early Treatment Diabetic Retinopathy Study (ETDRS) is composed of 10 Sloan letters, which are not used in the Greek, Cyrillic, and Central European alphabets. In this study we evaluate a modified ETDRS chart, the University of Crete (UoC) chart, which contains a set of letters readable by all European citizens.
Methods: In the UoC charts, the letters C, D, R, N, V, S, and Z were substituted with E, P, B, X, Y, A, and T, respectively. The similarity between the modified and the standard acuity charts was evaluated using two procedures. First, VA of 227 secondary school children (454 eyes) was evaluated using both sets of charts. Second, the relative difficulty for the identification of individual Sloan letters used in both charts, as well as letter M, was assessed from psychometric functions for five subjects.
Results: Bland-Altman plots revealed no statistical significant differences in the value of VA between the standard and the UoC set of charts. Although, estimates of identification log MAR threshold showed relatively significant interletter variability, in total, the new set of Sloan letters was equally identifiable with the original set.
Conclusions: The overall pattern of results suggests that the modified log MAR UoC charts forms a valid alternative to the ETDRS for assessing VA in multinational clinical trials, offering the advantage of containing letters recognizable by a wider population basis, such as European citizens, as well as subjects from countries using the Cyrillic alphabet.