Identification of keratoconus has become a step of primary importance in the preoperative evaluation for the refractive surgery. In order to decrease the secondary ectasia after refractive surgery the preoperative screening of keratoconus is very important. Corneal topography, the most useful tool in the diagnosis of keratoconus, offer us the possibility of an early diagnosis of KC by a lot of mathematical indices calculate to determine the disease with a higher specificity and sensitivity. Although useful, these indices may be difficult to understand and use.