Interobserver reliability of the prism and alternate cover test in children with esotropia

Objective: To determine 95% limits of agreement on a measurement and on a difference between 2 measurements for the prism and alternate cover test (PACT) at distance and at near fixation. Methods: In a test-retest reliability study, 143 children aged 60 months or younger with esotropia were examined by 2 masked examiners on 1 or 2 occasions yielding 199 test-retest pairs for PACT at distance fixation and 239 test-retest pairs for PACT at near fixation. Results: For angles greater than 20 prism diopters (PD), the 95% limits of agreement on a measurement and on a difference between 2 measurements were ±7.3 PD and ±10.4 PD, respectively, for PACT at distance and ±8.3 PD and ±11.7 PD, respectively, for PACT at near. For angles of 10 to 20 PD, the 95% limits of agreement on a measurement and on a difference between 2 measurements were ±4.1 PD and ±5.8 PD, respectively, for PACT at distance and ±3.3 PD and ±4.7 PD, respectively, for PACT at near. Conclusion: In childhood esotropia, differences of 12 PD or more for angles greater than 20 PD and differences of 6 PD or more for angles between 10 PD and 20 PD are likely to indicate real change. Smaller differences could be real change but could also be due to measurement error.