Repeatability of near visual acuity measurement at high and low contrast

The aim of this study was to determine the repeatability of high- and low-contrast visual acuity (VA) measurements at near.

Fifty-five normal subjects were recruited. Inclusion criteria included visual acuity of at least 0.00 logarithm of minimum angle of resolution (logMAR) on each eye at distance. One eye was selected for this study, either the one with a better acuity or randomly chosen if there was no difference between the two eyes. Near VA was measured in a random order with the PolyU high-contrast (PolyU-HC), the PolyU low-contrast (PolyU-LC), the Precision high-contrast (P-HC) and the Precision low-contrast (P-LC) charts at 400 mm. Measurements were repeated after one to two weeks. Repeatability was presented using the 95% limits of agreement between visits.

The between-visit repeatability was +/-0.063 logMAR for high-contrast and +/-0.141 for low-contrast using the PolyU charts. The between-visit repeatability was +/-0.120 logMAR for high-contrast and +/-0.110 for low-contrast using the Precision charts. Seventeen subjects had high-contrast VA better than -0.10 logMAR using Precision chart, which could not be measured by PolyU chart. The mean difference between high- and low-contrast VA was 0.108 from the Precision charts (median difference of 0.10 or one line).

The Precision charts could measure high-contrast near VA to threshold level. Practitioners should be aware of a VA difference of more than one line in repetitive measurement, at both high and low contrast. A difference in near high- and low-contrast VA of more than one line may warrant further investigation.