Influence of disease severity and optic disc size on the diagnostic performance of imaging instruments in glaucoma

PURPOSE. To evaluate the influence of disease severity and optic disc size on the diagnostic accuracy of three imaging technologies for structural assessment in glaucoma: confocal scanning laser ophthalmoscopy, scanning laser polarimetry, and optical coherence tomography. METHODS. One hundred five patients with glaucoma and 61 normal subjects were recruited from the Diagnostic Innovations in Glaucoma Study (DIGS). All individuals underwent imaging with the GDx Variable Corneal and Lens Compensator (VCC; Carl Zeiss Meditec, Inc., Dublin, CA), the Heidelberg Retina Tomograph II (HRT II; Heidelberg Engineering, GmbH, Dossenheim, Germany), and the Stratus OCT (Stratus OCT; Carl-Zeiss Meditec, Inc.) within a 6-month period. Severity of disease was based on the AGIS (Advanced Glaucoma Intervention Study) visual field score. To evaluate the influence of severity of glaucoma and optic disc size on the diagnostic accuracy of the imaging instruments, the sensitivities of the tests were fitted as a function of the AGIS score and disc area, by using logistic marginal regression models. RESULTS. The severity of visual field loss had a significant influence on the sensitivity of all imaging instruments. More severe disease was associated with increased sensitivity. This influence was similar among the three instruments. With regard to optic disc area, larger optic discs were associated with decreased sensitivity for the Stratus OCT parameter Average Thickness and the GDx VCC parameter Nerve Fiber Indicator, whereas small optic discs were associated with increased sensitivity. For the HRT II parameter Moorfields regression analysis classification, an inverse effect was observed. CONCLUSION. The diagnostic performances of the GDx VCC, HRT II, and Stratus OCT were significantly influenced by the severity of the disease and optic disc size. These covariates should be taken into account when comparing the performances of these tests for glaucoma diagnosis.