Comparison of the Ratio Between Anterior and Posterior Curvature Centered on the Thinnest Point (A/B Ratio) in Normal and Keratoconus Eyes.

Purpose: The purpose of this study was to evaluate in a large multi-national database (3873 eyes) the anterior to posterior radius of curvature ratio taken from a 3.0 mm optical zone centered on the thinnest point (A/B ratio) and determine its clinical applicability in discriminating healthy from ectatic eyes. Methods: Databases from three keratoconus centers (Homburg, Germany, Mashhad, Iran, and Rio de Janeiro, Brazil) were analyzed. Databases included healthy (n = 2061) and keratoconus eyes (n = 1812). Anterior and posterior radius of curvature values were determined by the "A" and "B" parameters from the Belin ABCD staging. The ratio was calculated by dividing the "A"/"B" parameter in radius of curvature. Results: Diagnostic accuracy expressed by area under the curve (AUC) was 0.951. Threshold value for classifying individuals as positive or negative was >1.27 with sensitivity of 85.8% and specificity of 95.2%. Conclusions: The A/B ratio centered on the corneal thinnest point shows high specificity, and moderate sensitivity. In the database of nearly 4000 eyes only 2 healthy eyes had an A/B ratio greater than 1.300, and no healthy eyes were above 1.335. An A/B ratio greater than 1.300 was highly suggestive of variance from normal with a specificity of 99.9%. Our results suggest that the A/B ratio may be a useful clinical tool in determining variance from normal. Translational Relevance: The A/B ratio centered on the thinnest point offers a simple, clinically applicable parameter with high specificity for detecting deviations from healthy corneal morphology. Its use may enhance early identification of ectatic disease and support more precise screening in refractive surgery candidates.