Defining real change in prism-cover test measurements.

Purpose: To describe the interobserver test-retest variability of both simultaneous prism and cover testing (SPCT) and alternate prism and cover testing (APCT) in horizontal deviations, and to calculate 95% limits of agreement that might be used to define real change.
Design: Prospective cohort study.
Methods: Twenty-three patients with sixth nerve palsy and three controls were independently examined by two experienced strabismus surgeons. SPCT and APCT were performed at distance and near fixation. Test-retest variability and agreement between tests were evaluated using Bland-Altman plots and 95% limits of agreement were calculated.
Results: For SPCT, the 95% limits of agreement half-widths were 6.3 prism diopters (pd) at distance fixation and 6.9 pd at near. For APCT, the 95% limits of agreement half-widths were 10.2 pd at distance and 9.2 pd at near.
Conclusions: Based on 95% limits of agreement half-widths between two examiners, a change in strabismus measurements of less than 10 pd may be attributable to test-retest variability. Changes of 10 pd or more are likely to represent real change and might be used as the threshold for management decisions.