Purpose: To report factors influencing the relationship between visual acuity (VA) fractions measured on Snellen chart and letter scores on the Early Treatment Diabetic Retinopathy Study (ETDRS) chart.
Design: This was a retrospective review from a single ophthalmology outpatient clinic.
Methods: All patients had routine consecutive VA testing in the right eye using a Snellen chart (1-6 m) and the ETDRS chart (4 m and/or 1 m), by the same optometrist, using a standardized testing protocol for each chart. Both acuity fractions and letter scores were converted to their equivalent logarithm of minimum angle of resolution (logMAR) for comparison. Multiple regression analysis was performed.
Results: A total of 237 patients with a wide range of ocular disease and VAs were enrolled. Mean age was 63 years (range, 18-95 years). Recorded VA (logMAR) was better on Snellen chart by a mean (95% limits of agreement) of -0.07 (-0.33 to +0.18, P < 0.001). There was a trend for subjects with VA worse than 6/36 to read 3 to 6 letters more and those better than 6/36 to read 4 to 5 letters less on the ETDRS chart. Visual acuity level and amblyopia contributed to a small portion (17%-26%) of the total variance in the difference between logMAR equivalents derived from the 2 charts.
Conclusions: The discrepancy in VA derived from Snellen and ETDRS charts was nonuniform across VA range. This has implications on interpretation of published studies converting Snellen fractions to logMAR for analysis and reporting of VA outcomes.