Strabometry precision: intra-examiner repeatability and agreement in measuring the magnitude of the angle of latent binocular ocular deviations (heterophorias or latent strabismus).

Background and purpose: This study was designed to compare heterophoria measurements obtained using the methods: prisms cover test (prism alternate cover test), von Graefe technique, Maddox rod test and modified Thorington test. Given the different methodological features of these tests, our working hypothesis was that these tests would not be interchangeable and repeatability would vary.

Methods: Horizontal deviation measurements were made at: far distance (six meters) and near distance (40 centimeters) on two occasions in 61 young subjects of mean age 19.7 years (range 18 -32 years), with essentially normal eyes and vision, who were not familiar with the methods used. Statistical repeatability and agreement were determined using the Bland and Altman method.

Results: Repeatability: No difference between the results of the various heterophoria tests was statistically significant. Coefficients of repeatability were always best when the tests were conducted at far, the cover test being the most repeatable. Agreement: Mean differences between absolute values ranged from 1.7 prism diopters to 5.1 prism diopters for measurements at far and from 2.1 prism diopters to 3.4 prism diopters at near.

Conclusions: The alternating prism cover test was the most repeatable test for measuring latent horizontal deviations. Among the subjective tests, the modified Thorington test was the most repeatable. The low level of agreement observed between the different tests makes their interchangeable use in clinical practice not recommended.