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T.T. McMahon, L. Szczotka–Flynn, J.T. Barr, R.J. Anderson, M.E. Slaughter, J.H. Lass, S.K. Iyengar, The CLEK Study Group; A New Method for Grading the Severity of Keratoconus: The Keratoconus Severity Score (KSS) . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1310.
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To determine the reliability and validity of a new method for grading the severity of keratoconus, the Keratoconus Severity Score (KSS).
The KSS is a 0–5 point severity scale using common clinical markers plus two corneal topographic indices. An initial test set of 1,012 eyes, including normal eyes, eyes with abnormal corneal and topographic findings but not keratoconus and eyes with keratoconus having a wide range of severity, was used to determine cut points for the KSS. Validation set 1 comprised of data from 128 eyes was graded by KSS and compared to a "clinicians ranking" of severity to determine if the scale fairly represented how a clinician would grade disease severity. Kappa statistics, sensitivity and specificity were calculated. A computer program was developed to automate the determination of the score. This was tested against a manual assignment of KSS in 2,121 (validation set 2) eyes from the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study as well as normal eyes and abnormal eyes but without keratoconus. Ten percent of eyes underwent repeat manual assignment of KSS in order to determine the variability of manual score assignment.
The KSS utilizes two corneal topography indices, Average Corneal Power and RMS error for higher order (Zernike terms) first surface wavefront error. Clinical signs including Vogt’s striae, Fleischer’s rings, and corneal scarring, and a manual interpretation of the map pattern were included to increase reliability. Validation set 1 yielded a kappa statistic of 0.90, with sensitivities ranging from 0.64 to 1.00 and specificities ranging from 0.93 to 0.98. The sensitivity and specificity for determining non–keratoconus from keratoconus were both 1.00. Validation set 2 demonstrated kappa statistics of 0.94 and 0.95 for right and left eyes, respectively. Test–retest analysis yielded kappa statistics of 0.84 and 0.83 for right and left eyes, respectively.
A simple and reliable grading system for keratoconus was developed that takes into account all grades of severity, including end–stage disease, minimzes misclassification and can largely be automated. Such a grading scheme could be useful in genetic studies for a complex trait such as keratoconus requiring a semi–quantitative measure of disease presence and severity.
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