Between-Subject Variability in Healthy Eyes as a Primary Source of Structural–Functional Discordance in Patients With Glaucoma

PURPOSE: To test with an independent data set the finding that between-subject variability in healthy eyes is the primary source of structural–functional discordance in patients with glaucoma. METHODS: Neuroretinal rim area, retinal nerve fiber layer thickness, and perimetric data were analyzed for one eye in each of 55 control subjects and for 245 right eyes of patients in the United Kingdom Glaucoma Treatment Study. Data were gathered with the Heidelberg Retina Tomograph (HRT), Stratus Optical Coherence Tomograph (OCT), and Humphrey Field Analyzer (HFA). Discordance was quantified as width of the limits of agreement from a Bland-Altman analysis of depth of defect. The ratio of variances (F test) for the patient and control groups was computed for comparisons of HFA-OCT, HFA-HRT, and OCT-HRT. Bonferroni adjustment required P less than 0.017 for statistical significance. The discordance in the patients was also quantified as the 95% prediction interval computed from the discordance in controls using the Hood-Kardon model for the HFA-OCT comparison. RESULTS: The F ratio comparing discordance in patients and controls was 0.77, 1.43, and 1.32 for the HFA-OCT, HFA-HRT, and OCT-HRT comparisons with P values 0.88, 0.06, and 0.11, respectively. For the Hood-Kardon model, 4.7% of the patients had discordance outside the 95% prediction interval computed from the discordance in controls. Similar results were obtained when all comparisons were repeated for left eyes of patients. CONCLUSIONS: These results confirm previous findings that between-subject variability in healthy eyes is the primary source of structural–functional discordance in patients with glaucoma, and extends this finding to a structural–structural comparison.