Recently some of my patients have expressed concerns at the end of the examination that they couldn’t read the bottom line on the letter chart I use to measure visual acuity. I’m never sure what to say to them. Can you help?

In my experience, the most common form of visual acuity letter chart has a bottom line that measures 6/5 (20/17). Some patients can with the optimum refractive correction read 6/5 with each eye in turn and others only with both eyes together.

When I am assessing visual acuity in order to check for disease or determine the optimum refractive correction, I push patients down the letter chart to the threshold (until they make mistakes) and explain that the test is more accurate when they get a few letters wrong and it’s quite normal to get a few letters wrong. Everybody does.

However, when I have determined the optimum refractive correction, I ask the patient to read with both eyes a line of letters above the one that they struggled with during the refraction stage. So, if they struggled with 6/5, I ask them to read 6/6 and they usually read all or most of these correctly. If they struggled with 6/6 during refraction, I ask them to read 6/9 (or 6/7.5 if I have this line) I advise that this is the line I would expect them to read for the age group they are in (assuming there is no eye disease present).

This way they finish the refraction part of the eye examination by reading a complete line of letters (or almost complete) easily and quickly.

In my experience, it’s better for the patient to finish reading letters well than for them to finish struggling to read letters.