Abstract Background : Keratoconus (KC) is a bilateral asymmetric localised inferior and central corneal thinning and ectasia. Usually, it causes high myopia and irregular astigmatism resulting in a poor quality of vision. One of the most important ways of avoiding iatrogenic ectasia in cornea during refractive surgery is to detect KC in its earliest stage using the Pentacam. An important asymmetry decentration index is the inferior minus superior (IS) value. Purpose : Detection of Inferior minus Superior (IS) value and its Cut-off Value Methods : This is a cross-sectional, case control study that included 92 right eyes of 92 subjects, who presented to the outpatient clinic of “Al Watany Eye Hospitals” seeking refractive surgery or routine medical consultations. Our study focused on the measurement of the IS-value in early KC (TKC grade 1 and FFKC) and normal eyes. The right eye of each candidate was scanned 5 times in the same setting using the Pentacam. The study adhered to the tenets of the Declaration of Helsinki, and an approval was obtained from the ethical committee of Ain Shams University Hospitals. Results : We detected remarkably high statistically significant difference between patients and controls regarding the IS-value (p < 0.001). The mean (±SD) of control and cases group was 0.3 ± 0.2 and 1.4 ± 0.6, respectively. The cut-off point of 0.69 with high area under the receiver operating characteristic (AUROC). Coefficient of variation median (95% CI) for cases and controls were 1.4% (0.9 to 3.1%) and 6.2% (3 to 9.4%), respectively. Conclusion : The IS-value has a high repeatability and AUROC for differentiating early KC from normal corneas. It can be regarded as an accurate parameter for early KC detection.