To compare posterior corneal mean power and astigmatism in normal versus keratoconus-affected eyes and determine their optimal cutoff points to maximize sensitivity and specificity in discriminating keratoconus from normal corneas.

A total of 204 normal eyes and 142 eyes with keratoconus were enrolled.Posterior corneal mean power and astigmatism were measured using the Galilei double Scheimpflug camera and compared between the normal and keratoconus groups. Receiver operating characteristic curves were used to compare the sensitivity and specificity of the measured parameters and to identify optimal cutoff points for discriminating keratoconus from normal corneas.

Posterior corneal mean power was -6.290.20 D in the normal group and -7.770.87 D in the keratoconus group (P<0.001). The mean magnitude of the posterior corneal astigmatism was -0.32±0.15 D and -0.94±0.39 D, respectively (P<0.001).With-the-rule, against-the-rule, and oblique astigmatism of the posterior corneal surface was found in 1.0%, 97.0%, and 2.0% of eyes in the normal group and in 2.8%, 79.7%, and 17.5% of eyes in the keratoconus group, respectively (P<0.001). The posterior corneal mean power and astigmatism were very strong to distinguish keratoconus from normal(area under the curve, 0.99 and 0.95, respectively). Optimal cutoff point for posterior corneal mean power was-6.70D(sensitivity, 93.7%; specificity, 99.5%). The value for posterior corneal astigmatism was -0.54 D(sensitivity, 88.0%; specificity, 91.7%).

Posterior corneal mean power and astigmatism measured by the Galilei analyzer can effectively discriminate keratoconus from normal corneas. The cutoff points provided in this study can be used in clinical settings for the screening of keratoconus.