Inter-observer agreement in clinical optic disc measurement using a modified 60 D lens

Abstract

Purpose: To assess the inter-observer agreement of the measurement of optic disc dimensions by two observers using a modified 60 dioptre (D) fundus examination lens. Method: The vertical disc and cup diameters of 29 eyes were measured by two independent observers using a 60 D lens modified by incorporation of a 0.1 millimetre scale graticule. The vertical cup/disc ratio was calculated. Inter-observer agreement was assessed by calculation of the inter-observer differences and by the weighted kappa statistic. Results: The two observers showed good agreement for the measurement of disc diameter (mean difference -0.04; range -0.04, 0.2) and for cup diameter (mean difference -0.03; range 0.3, 0.2). Closer agreement for the vertical cup/disc ratio was achieved (mean kappa 0.96; 95% confidence limits 0.90, 1.0). The 95% confidence limit for the mean inter-observer difference in cup/disc ratio was 0.11, suggesting that a change of > 0.1 in the assessment of the cup/disc ratio by this technique is significant at the 5% level. Conclusion: High inter-observer agreement of optic disc measurement can be achieved with this technique. The method has the potential to improve the clinical evaluation of the optic disc and the precision and accuracy of the clinical measurement of other fundal structures.