Visual acuities in patients with ARMD: Differences in Snellen and ETDRS measurements

Purpose. We undertook this study to assess the correlation between Snellen and ETDRS visual acuity measurements in patients with age-related macular degeneration (ARMD). Methods. We reviewed the charts of 37 consecutive patients (69 eyes) with ARMD in at least one eye. All patients had best-corrected visual acuities recorded using both the Snellen and ETDRS charts at the same visit Statistical analysis of differences in proportions was performed using the sign test. Results. Of the 69 eyes, Snellen acuity equaled ETDRS acuity in 20 eyes (29%). Of the 49 eyes in which the two measurements did not correlate, ETDRS acuity was better than Snellen acuity in 35 (71%), while ETDRS acuity was worse than Snellen in 14 (29%) (p=0.001). Among the eyes in which the two measurements were equal, the mean acuity was 20/50. For the group in which ETDRS overestimated Snellen acuity, the mean acuities were 20/300 by Snellen and 20/125 by ETDRS, with a mean difference of 4.0 lines. Among the eyes with better acuity by Snellen than by ETDRS, mean acuities were 20/125 by Snellen and 20/200 by ETDRS, with a mean difference of 1.6 lines. Conclusions. ETDRS acuity overestimated Snellen acuity in the majority of eyes; this group had the lowest mean acuity. These differences should be considered when applying results of studies based on ETDRS acuities to clinical practice.