To compare the vertical and horizontal cup-to-disc ratio (VCDR, HCDR) by an updated optical coherence tomography (OCT) Bruch membrane opening (BMO) algorithm and stereoscopic optic disc photograph readings by glaucoma specialists.

Reliability analysis.

A total of 195 eyes (116 glaucoma and 79 glaucoma suspect) of 99 patients with stereoscopic photographs and OCT scans of the optic discs taken during the same visit were compared. Optic disc photographs were read by 2 masked glaucoma specialists for VCDR and HCDR estimation. Intraclass correlation coefficient (ICC) and Bland-Altman plots were used to assess the agreement between photograph reading and OCT in estimating CDR.

OCT images computed significantly larger VCDR and HCDR than photograph reading before and after stratifying eyes based on disc size (P < .001). The difference in CDR estimates between the 2 methods was equal to or greater than 0.2 in 29% and 35% of the eyes for VCDR and HCDR, respectively, with a mean difference of 0.3 in each case. The ICCs between the readers and OCT ranged between 0.50 and 0.63. The size of disagreement in VCDR correlated weakly with cup area in eyes with medium (r² = 0.10, P = .008) and large (r² = 0.09, P = .007) discs.

OCT and photograph reading by clinicians agree poorly in CDR assessment. The difference in VCDR between the 2 methods was depended on cup area in medium and large discs. These differences should be considered when making conclusions regarding CDRs in clinical practice.