The most common clinical measurement of visual function is visual acuity, measured by means of a Snellen chart. Recently, its use has been expanded not only to measure visual acuity, but also to measure the progress of disease or the efficacy of therapeutic interventions. In a test-retest mode however, the main difficulty with a Snellen chart is the lack of a precise scoring system. Traditional line-by-line scoring typically requires only that 50-80% of the letters in a line be identified correctly in order to score a successful reading of the entire line. Thus, the variability in line-by-line scoring is inherently high. When letter-by-letter scoring (which inherently has a lower variability) is used, test-retest visual acuity measurements are known to be - up to a factor of two - more precise.
We have developed a unique method of scoring Snellen chart responses through the use of LogMAR transformations that account for each letter read. The final, cumulative letter-by-letter LogMAR value is converted into an effective Snellen fraction. The mathematical processes are embedded in an EXCEL-based tool and are transparent to the practicing clinician. This letter-by-letter, LogMAR-based, scoring tool is available from this web site. In addition, the tool has a similar ETDRS spread sheet for those who use ETDRS charts.
We have designed this tool for clinicians, who are untrained in LogMAR use, so that they can: 1) apply these visual acuity data as a refined diagnostic tool, 2) assess progression of eye disease, or 3) quantify the efficacy of treatment of certain pathological eye conditions with greater precision. Potentially, this tool has broad applications for clinicians who, for a variety of reasons, have been hesitant to incorporate LogMAR principles into their clinical patient databases. Moreover, this tool has possible beneficial medico-legal implications. It could protect clinicians who are struggling to quantify visual performance in a manner that is: a) subtle enough to have scientific credibility, b) sufficiently established to supply a defensible metric acceptable to scientists worldwide, and c) robust enough to stand up in courts of law.