Differences in visual acuity between the eyes: determination of normal limits in a clinical population.

We determined the difference in visual acuity between the right and left eyes of patients and also determined the maximal level of acuity expected in a group of visually normal individuals. Visual acuity was measured in the right and left eyes of 72 subjects aged from 16 to 67 years using Bailey-Lovie type charts. The chart was read until fewer than three letters were read on a line and acuities were based on each letter correct contributing -0.02 to the overall score. For 12 of these subjects, visual acuity was measured a further four times. Average visual acuities for right and left eyes were -0.137 and -0.126log MAR, respectively. The signed visual acuity difference was normally distributed and had a standard deviation of 0.050log MAR. The mean visual acuity difference for the 12 subjects for whom the measures were repeated was 0.033log MAR (SD of the signed visual acuity difference: 0.049log MAR). We conclude that if a patient has a difference in visual acuity between the two eyes of more than 5 letters on a Bailey-Lovie style chart, further investigation is indicated.