Agreement between three methods for measuring near point of convergence among patients with different refractive errors

Purpose: To describe the agreement of three methods of Near Point of Convergence (NPC) measurement among patients with different refractive errors.

Methods: 60 asymptomatic subjects, 18 - 25 yrs old, were included in 3 groups: emmetropes, myopes and hypermetropes. All subjects underwent NPC break point and recovery point measurement by Royal Air Force (RAF) rule, Pencil Rule (PR) and penlight with red green glasses (RG) using standard techniques. The values obtained were compared within each group by Friedman test. Bland Altman plots were constructed and Limits of Agreement calculated.

Results: Hypermetropes performed poorly in RG test with significantly receded break point and recovery point values (10.30 ± 1.45cm, 13.13± 1.20cm) compared to RAF test (7.18 ± 1.86 cm, 10.15 ± 2.11cm ) and PR test (7.78 ± 1.75 cm, 10.75 ± 1.44cm). The recovery point values of the emmetropes with RG test (10.15 ± 2.32cm) was significantly receded compared to PR (9.30 ± 1.72 cm) and RAF test (Emm: 9.08 ± 2.30cm). The myopes performed better with PR test with significantly better recovery point values with PR test (8.70 ± 1.97 cm) compared to RAF (9.68 ± 2.08) and RG (9.45 ± 1.73) tests. The limits of agreement were wide suggesting disagreement between the tests.

Conclusion: The RG test yields more receded results in hypermetropes compared to the RAF and PR tests, and the PR test yields better results than the RAF test in myopes. Thus, the results obtained by these different methods show a lack of agreement. The variability is not uniform in patients with different refractive errors.