Repeatability of near point of fixation disparity test

Vergence assessment is a fundamental component of binocular vision evaluation, enabling clinicians to detect and monitor conditions such as convergence insufficiency. Accurate and repeatable clinical tests are essential for reliable diagnosis and for tracking patient progress over time.

The Near Point of Fixation Disparity (NPFD), associated positive fusional vergence (APFV) range, and associated negative fusional vergence (ANFV) range are polarised Nonius-based tests designed to evaluate vergence system integrity. However, their repeatability remains uninvestigated, limiting clinical utility. This study is the first to assess intraexaminer and interexaminer repeatability of these tests in symptomatic and asymptomatic participants. The NPFD test is commercially available from Vision Assessment Corporation, Rolling Meadows, IL, USA.

This prospective, cross-sectional study enrolled 64 university students (32 symptomatic, 32 asymptomatic; aged 18-27 years) from King Saud University, Riyadh, Saudi Arabia. Symptom status was classified using the Convergence Insufficiency Symptom Survey (CISS; score ≥21 = symptomatic). Two trained optometrists performed all measurements across two sessions separated by two weeks. Repeatability was assessed using mean difference, 95% confidence intervals, intraclass correlation coefficient, coefficient of repeatability, and Bland-Altman analysis.

Intraexaminer repeatability of the NPFD test ranged from moderate to good with intraclass correlation coefficient 0.47-0.86, with the strongest agreement observed in asymptomatic participants. The APFV and ANFV ranges demonstrated poor intraexaminer repeatability, with limits of agreement frequently exceeding 10-15 prism dioptres. Interexaminer repeatability was poor across all three tests in both groups, with negative intraclass correlation coefficient values in several comparisons and limits of agreement exceeding 20 prism dioptres.

Although the NPFD test can achieve acceptable intraexaminer repeatability, its interexaminer reliability remains poor, raising concerns about its use as a diagnostic tool across different examiners. Vergence assessments should be performed by the same examiner, and changes of less than 10-15 prism dioptres should be interpreted with caution. Standardisation of testing protocols is strongly recommended to improve the clinical applicability of these tests.