Effect of scoring and termination rules on test-retest variability of a novel high-pass letter acuity chart

Test-retest variability (TRV) limits our ability to detect clinically significant changes in visual acuity (VA). We wanted to compare the effect of scoring and termination rules on TRV for logMAR charts, employing either conventional or pseudo high-pass (Vanishing Optotype) letters.

VA measurements and TRV were compared in 50 uncorrected normal observers (17 male, mean age 42.8 ± 16.2 years) using both conventional logMAR-style charts and letter charts of the same layout but containing pseudo high-pass letters (Moorfields Acuity Chart [MAC]). Additional charts employing a different 10-letter alphabet to the Sloan set were also tested. Mean spherical refractive error was -0.93 diopters (D; range, -5.38 to +3.00 D). Acuity scores were calculated using three methods: letter-by-letter, with either line- or chart-based termination, and line-by-line scoring. Bland-Altman methods were used to calculate 95% ranges for TRV.

While acuity thresholds were higher for the MAC, they were less affected by termination criteria and displayed significantly lower 95% TRV values across all scoring techniques. Ordinary least squares regression analysis confirmed a proportional as well as systematic bias between conventional and MAC measurements (r(2) = 0.217, P = 0.001) such that the difference between the two was greater with better VA.

TRV was consistently lower for a logMAR chart employing high-pass rather than conventional letters in uncorrected refractive error and was less affected by termination and scoring methods. The MAC was also less affected by optical defocus. Further work is required to determine the usefulness of different charts to differentiate between optical and neural losses of vision.