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The inferior-superior (I-S) index is calculated as the difference between inferior and superior average dioptric (D) values. The average of five superior points (in blue) above the horizontal meridian are compared to the average of five inferior (in orange) points. The points are spaced in 30-degree intervals and are approximately 3.0-mm from the corneal vertex. Any value >1.4 is suggestive of keratoconus [11, 44].
