To determine cutoffs for interocular differences in ganglion cell–inner plexiform layer thickness in normal healthy eyes and to evaluate the diagnostic performance of these values for differentiating between normal subjects and glaucoma patients.

Observational, cross-sectional study.

Macular and optic disc scanning were performed in 275 normal subjects, 58 glaucoma patients, and 58 normal controls by high-definition optical coherence tomography. The ganglion cell–inner plexiform layer thickness was calculated, and the normal ranges of the interocular differences were determined as 2.5th and 97.5th percentiles. The signed and absolute interocular differences were compared between normal subjects and glaucoma patients.

The mean ± standard deviation interocular difference in the average ganglion cell–inner plexiform layer thickness thickness was 0.10 ± 2.31 μm, which was not statistically significant (P = .466). The 2.5th and 97.5th percentiles of the interocular difference were −4.10 μm and +5.00 μm, respectively. On multiple regression analysis, the interocular difference in axial length was correlated with the interocular difference in average ganglion cell–inner plexiform layer thickness thickness (β = 2.044, P = .003). The signed and absolute interocular differences in ganglion cell–inner plexiform layer thickness were higher in glaucoma patients than in normal subjects (all P < .001). Sensitivity and specificity of absolute interocular differences ranged from 25.9% to 51.7% and from 93.1% to 100.0%, respectively.

Ganglion cell–inner plexiform layer thickness shows significant interocular symmetry in normal subjects. An absolute interocular difference exceeding normal limits may be indicative of glaucoma.