The intraexaminer and interexaminer repeatability of the alternate cover test using different prism neutralization endpoints

Abstract

Purpose: The alternate cover test (ACT) measures the magnitude of a deviation, but different prism endpoints may be used. This study investigated the intraexaminer and interexaminer repeatability of the ACT using two different prism neutralization endpoints.

Methods: To determine repeatability of two prism neutralization endpoints (first neutral and midpoint of reversal), two experienced examiners measured near point phoria on 72 subjects. Measurements were repeated with examiners masked to each other and the first results. Paired t-tests were used to assess interexaminer and intraexaminer repeatability and the agreement between different prism endpoints for each examiner within each session. Signed and absolute differences in phoria magnitude were calculated.

Results: For two different test sessions with the same examiner using the same prism neutralization endpoint (intraexaminer repeatability), none were significant except examiner 1 using first neutral (p = 0.038). Average signed differences (ASD's) between two test sessions for each comparison were <0.5 prism diopter (pd) with SD's of </=2.56 pd. Absolute differences ranged from 1.30 to 1.89 pd. When two different examiners used the same prism endpoint within the same session (interexaminer repeatability), there were no significant differences between examiners. ASD's between the two examiners for each comparison were </=0.53 pd with SD's </=2.03 pd. The absolute differences ranged from 1.19 to 1.67 pd. Comparisons of the two different prism endpoints by the same examiner at a single test session were all significantly different (p < 0.005), but the differences (<1 pd between the two endpoints) were not clinically significant. ASD's were <1 pd with SD <2 pd. Absolute differences ranged from 1 to 1.58 pd.

Conclusions: With experienced examiners, either prism endpoint provides high interexaminer and intraexaminer repeatability (</=0.5 pd). Although the two prism endpoints differ statistically, the differences are not clinically significant.