Abstract
AIMS: To assess the intraobserver agreement, interobserver agreement, and the agreement between a digital stereo optic disc camera (Discam) and Heidelberg retina tomograph (HRT) in measuring area cup-disc ratio (ACDR) and radial cup-disc ratio (RCDR) by two observers. METHODS: The optic discs of 78 eyes of 39 people (17 cases of primary open angle glaucoma, eight normal tension glaucoma, two ocular hypertension, and 12 normal subjects) were imaged with Discam and HRT. Two observers independently drew the disc margins on the HRT mean topography images and the disc and cup margins on the Discam images. ACDR and the RCDR at various angles were measured with the two systems. Intraobserver agreement, interobserver agreement, and the agreement between the two systems were assessed by 95% tolerance limit of changes (TC) and intraclass correlation coefficient (ICC). RESULTS: Eight eyes were excluded due to poor image quality (six Discam and two HRT). 70 eyes were analysed. The intraobserver ACDR agreement was almost perfect in both systems (ICCs = 0.97 and 0.92, and TCs = 11.0% and 15.1% in HRT and Discam respectively). The interobserver ACDR agreement was almost perfect in HRT (ICC = 0.97) and substantial in Discam (ICC = 0.79), (TCs = 10.5% and 24.5% respectively). The ACDR agreement between the two systems was substantial in observer A (ICC = 0.67) and moderate in observer B (ICC = 0.53), (TCs = 24.8% and 46.7% respectively). The HRT measured the ACDR significantly larger than the Discam (p <0.001), and the differences were significantly larger in the glaucomatous group (p <0.001). RCDR agreement between the two systems was fair to substantial in observer A (ICC = 0.36 to 0.74) and slight to moderate in observer B (ICC = 0.12 to 0.45). Both observers achieved the best RCDR agreement between the two systems at the inferior optic disc position. CONCLUSION: There is almost perfect intraobserver agreement in each system. The interobserver agreement was better with the HRT than the Discam. There was substantial variation in ACDR and RCDR agreement between the two systems measured by the two observers. The variation in ACDR and RCDR measurements between the two systems may be too large for interchangeable use in a clinical setting.