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Cornea. 1998 May;17(3):267-77.

Repeatability and agreement of two corneal-curvature assessments in keratoconus: keratometry and the first definite apical clearance lens (FDACL). CLEK Study Group. Collaborative Longitudinal Evaluation of Keratoconus.

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  • 1Southern California College of Optometry, Fullerton, California, USA.



This study was conducted to determine the agreement and test-retest repeatability of two methods for measuring corneal curvature in keratoconus: keratometry and the First Definite Apical Clearance Lens (FDACL). Our interest in the FDACL procedure stems from the important contact lens-fitting information and documentation of disease progression provided by the FDACL trial lenses and observation of fluorescein patterns.


The Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is an observational study that has enrolled 1,209 keratoconus patients to characterize the progression of keratoconus, to determine factors associated with its progression, and to assess its impact on quality of life. Ten percent of the patients were randomly selected at baseline for a retest examination. The baseline examination, which included keratometry and FDACL, was repeated in this sample. The FDACL is the flattest lens in the standardized CLEK trial lens set that vaults the apex of the cone. FDACL provides an estimate of the sagittal height of the cone.


The correlation of FDACL with the steep keratometric reading (r = 0.89; p = 0.0001) and the flat keratometric reading (r = 0.83; p = 0.0001) were high. Test-retest repeatability as measured by the intraclass correlation coefficient (ICC) was high: FDACL ICC, 0.97; steep keratometric reading ICC, 0.96; and flat keratometric reading ICC, 0.95. Test-retest repeatability of FDACL remained high in advanced disease.


FDACL provides a repeatable new procedure for determining disease severity in keratoconus.

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