Objective To evaluate the ability of parameters measured by three-dimensional (3D) corneal and anterior segment optical coherence tomography (CAS-OCT) and a rotating Scheimpflug camera combined with a Placido topography system (Scheimpflug camera with topography) to discriminate between normal eyes and forme fruste keratoconus.

Methods Forty-eight eyes of 48 patients with keratoconus, 25 eyes of 25 patients with forme fruste keratoconus and 128 eyes of 128 normal subjects were evaluated. Anterior and posterior keratometric parameters (steep K, flat K, average K), elevation, topographic parameters, regular and irregular astigmatism (spherical, asymmetry, regular and higher-order astigmatism) and five pachymetric parameters (minimum, minimum–median, inferior–superior, inferotemporal–superonasal, vertical thinnest location of the cornea) were measured using 3D CAS-OCT and a Scheimpflug camera with topography. The area under the receiver operating curve (AUROC) was calculated to assess the discrimination ability. Compatibility and repeatability of both devices were evaluated.

Results Posterior surface elevation showed higher AUROC values in discrimination analysis of forme fruste keratoconus using both devices. Both instruments showed significant linear correlations (p<0.05, Pearson's correlation coefficient) and good repeatability (ICCs: 0.885–0.999) for normal and forme fruste keratoconus.

Conclusions Posterior elevation was the best discrimination parameter for forme fruste keratoconus. Both instruments presented good correlation and repeatability for this condition.