To estimate the sources of variation for Pigment epithelium central limit-Inner limit of the retina Minimal Distance averaged over 2π (PIMD-2π), and further to analyse their consequences for clinical measurements of glaucoma.Forty subjects with early to moderate stage glaucoma were included. Three SD-OCT volumes of the optic nerve head (ONH) were captured at two occasions. Each volume was segmented three times for PIMD-2π. The magnitude of the sources of variation for PIMD-2π measurements was estimated with an analysis of variance.A 95% confidence interval for mean PIMD-2π was estimated to 215 ± 12 μm (df = 38). The estimated variance for subjects was 1280 μm . The within-subject estimated variance for occasions, volumes and segmentations was 10 μm , 30 μm and 40 μm , respectively. The within-subject variances were used to model follow-up of PIMD-2π over time. A linear loss rate of 0.05 of baseline PIMD-2π/year was assumed. A significant PIMD-2π change could be detected in approximately 16-18 months with evenly spaced visits every 4 or 6 months.Due to the small within-subject estimated variances, a clinically undesirable PIMD-2π change from baseline can be detected in approximately 18 months. Detection of significant PIMD-2π loss in a subject requires knowledge of normal age loss and measurement variability.