The LogMAR chart is designed to enable more accurate estimates of acuity as compared to other acuity charts.
Each line of the LogMAR chart comprises the same number of test letters (effectively standardizing the test across letter size); letter size from line to line varies logarithmically, as does the spacing between lines (making the chart easy to use at nonstandard viewing distances). Furthermore, the final LogMAR score is based on the total of all letters read.
Each letter has a score value of 0.02 log units. Since there are 5 letters per line, the total score for a line on the LogMAR chart represents a change of 0.1 log units. The formula used in calculating the score is:
- LogMAR VA = 0.1 + LogMAR value of the best line read – 0.02 X (number of letters read)
The ETDRS test incorporates specific design criteria to make it more accurate than the Snellen or Sloan acuity tests. These include:
- Same number of letters per row (five letters per row)
- Equal spacing of the rows on a log scale (the rows are separated by 0.1. log unit)
- Equal spacing of the letters on a log scale
- Individual rows balanced for letter difficulty
To prevent memorization, different versions of the ETDRS test chart are available. The three standard versions of the ETDRS chart are R, 1 and 2.
To properly evaluate ETDRS, the test should be conducted under standardized lighting conditions. ESV-3000 are the devices that self-standardizes the test lighting level to the recommended photopic test level of 85 cd/m2 , or a mesopic level of 3 cd/m2. These light levels are recommended by the National Academy of Sciences’ Committee for Vision Testing Standards and are required by the FDA for ETDRS evaluation in clinical trials.
Scoring the ETDRS Chart
ETDRS scoring can be accomplished in a number of different ways. The two most common methods are described below. Both of these methods provide scores that can be used for statistical analysis, unlike the scores provided by the standard Snellen or Sloan Acuity tests.
ETDRS Scoring Method 1:
The patient starts are the top of the chart and begins to read down the chart. The patient reads down the chart until he or she reaches a row where a minimum of three letters on a line cannot be read. The patient is scored by how many letters could be correctly identified.
ETDRS Scoring Method 2:
The ETDRS charts were originally used in ETDRS studies where patients had relatively poor vision. For these studies, starting patients at the top of the chart to read down worked well since most patients could read only about half way down the chart.
However, the ETDRS charts are now being widely used for many studies, including IOL and refractive surgery studies, where patients have very good vision. For these patients, it makes little sense to start them at the top of the chart and read down. For these patients, a second scoring method is used.
The patient starts on the last row where he or she can read all of the letters, and then reads down until he or she reaches a row where a minimum of three lines cannot be read. For these patients, a decimal ETDRS acuity score can be used.
How Are Bailey-Lovie Charts and ETDRS different?
Bailey-Lovie charts incorporate the European design of LogMAR; i.e. letter sizes are rectangular. For example, for the Bailey-Lovie Chart Design, a 20/20 letter is 4 minutes of arc in height by 5 minutes of arc in width. ETDRS letters are all square, i.e. 5 by 5.