Visual Acuity: Comparison of ETDRS Charts Using Latin Letters, Tumbling E, or Landolt C Optotypes

Visual acuity (VA) is a key outcome measure in clinical practice, epidemiological studies, and in clinical trials. However, letter-based charts may disadvantage individuals who are illiterate or are unfamiliar with Latin text, thereby limiting subject participation and complicating comparisons across studies. We compared VA using Early Treatment Diabetic Retinopathy Study (ETDRS) charts for Latin letters, Landolt-C, or Tumbling-E optotypes to determine their association and measurement variability.

Seventy-eight patients from an eye clinic underwent VA testing using the various optotypes on an ETDRS logMAR chart. VA was measured with a pinhole (PH), uncorrected, and corrected for the three optotypes, and compared with results in severe VA loss. Regression slopes, coefficients of determination, and limits of agreement (LoA) were determined.

All three optotype VAs showed a similar response to PH use, with an average improvement of approximately three logMAR lines. For uncorrected VA, Tumbling-E and Landolt-C showed slopes near 1.0 relative to letter VA across a broad range with R2 > 0.93. For corrected VA, slopes were 0.73 (Tumbling-E) and 0.58 (Landolt-C), with R2 ≈0.70. Combining our data with prior studies extended the range to 20/2000 for Landolt C and letter VA, yielding slopes near 1.0 and R2 > 0.94. LoA across comparisons ranged from logMAR ±0.12 to ±0.23.

Non-letter optotypes provide clinically useful and broadly comparable measures of VA at the population level supporting their use in clinical practice or population/clinical studies. However, the wide variability means they cannot be considered interchangeable for individual subjects.