A Clinically Useful Tool to Determine an Effective Snellen Fraction: Details

A patients visual function has been routinely assessed by a visual acuity measurement, usually by means of a Snellen chart. More recently, visual acuity measurements have been extended to measure the progress of disease or the efficacy of therapeutic interventions. Thus, accurate and reproducible visual acuity measurements are needed. However, the intrinsic variability in line-by-line scoring is high because only 50-80 of the letters in a line need be correctly identified to score a successful reading of the entire line. To reduce this high test-retest variability, we developed a tool whereby the cumulative letter-by-letter logarithm of minimum angle of resolution LogMAR values of a patients responses are converted into an effective Snellen fraction. With these concepts, test-retest visual acuity measurements are known to be more precise by up to a factor of 2. These concepts have been combined into a spreadsheet that automatically and transparently calculates the effective Snellen fraction. This spreadsheet is simple to use, making its introduction into the clinic straightforward. This report documents the details of the processes constituting this tool, which is available for download from the Pennsylvania College of Optometry web site www.salus.edu.