An optic disc grid: its evaluation in reproducibility studies on the cup/disc ratio.

A grid system is described which may be superimposed upon stereoscopic pairs of optic disc photographs. This grid system allowed reproducible inter- and intraobserver measurements of the cup/disc (C/D) ratio. The surface contour of the optic disc was not clear-cut in approximately 10% of the population studied, which led to a reduction in reproducibility of the measurement made within the group as a whole. As a result it was found that a large increase in the C/D ratio would have to occur for such change to be of statistical significance. Even with an optic grid as a reference system the serial measurements of the C/D ratio are unlikely to be of value in the management of patients with chronic glaucoma.