Interobserver agreement on visual field progression in glaucoma: a comparison of methods

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Abstract
Aim: To examine the level of agreement between clinicians in assessing progressive deterioration in visual field series using two different methods of analysis. Methods: Each visual field series satisfied the following criteria: more than 19 reliable fields, patient age over 40 years, macular threshold at least 30 dB. The first three fields in each series were excluded to minimise learning effects: the following 16 were studied. Five expert clinicians assessed the progression status of each series using both standard Humphrey printouts and pointwise linear regression (progressor). The level of agreement between the clinicians was evaluated using a weighted kappa statistic. Results: A total of 432 tests comprising 27 visual field series of 16 tests each were assessed by the clinicians. The level of agreement on progression status between the clinicians was always higher when they used progressor (median kappa = 0.59) than when they used Humphrey printouts (median kappa = 0.32). This was statistically significant (p = 0.006, Wilcoxon matched pairs signed rank sum test). Conclusions: Agreement between expert clinicians about visual field progression status is poor when standard Humphrey printouts are used, even when the field series studied are long and consist solely of reliable fields. Under these ideal conditions, clinicians agree more closely about patients’ visual field progression status when using progressor than when inspecting series of Humphrey printouts.