Calculating reading acuity
An estimate of reading acuity is given by the smallest print size at which the patient can read the entire sentence without making significant errors. (Usually reading performance deteriorates rapidly as the acuity limit is approached, and it is easy to determine the level where reading becomes impossible). This method measures acuity to the nearest 0.1 logMAR.
The MNREAD ACUITY CHARTS can be used to provide a more sensitive and reliable measure of reading acuity. Each sentence has 60 characters, which corresponds to 10 standard length words, assuming a standard word length of 6 characters (including a space). Thus, each sentence can be divided into 10 smaller parts, and acuity can be measured to the closest 0.01 logMAR.
Acuity = 1.4 - (sentences x 0.1) + (errors x 0.01).
- After the patient has read as much of the chart as possible, count the number of sentences that the patient read or attempted to read. If the patient did not start to read from the top of the chart, then include the sentences above the starting level as if they had been read.
- Count the number of words that the patient read incorrectly.
- Calculate reading acuity (in logMAR) using the following formula:
- See the example of this calculation.
Scoring for non-standard viewing distances
A convenient feature of the logMAR scale is that it allows simple conversion of reading acuity between different viewing distances.
When the chart is used at a distance other than 40cm, determine the patient's score as outlined in the previous section, then adjust this value to account for the viewing distance used.