sensitivity and specificity of galilei and orbscan in the diagnosis of subclinical keratoconus

purpose: to determine the sensitivity and specificity of posterior corneal elevation in discriminating keratoconus and subclinical keratoconus from normal corneas by galilei and orbscan ii diagnosed on the basis of topography. methods: three groups (keratoconus, subclinical keratoconus and normal) were evaluated with galilei and orbscan ii and results were compared to those obtained by gold standard placido disc-based topography. in one eye of each patient, posterior corneal elevation was measured in the central 3 mm zones using the galilei rotating double scheimpflug camera. posterior corneal elevation in keratoconus and subclinical keratoconus was compared to that in normal corneas. receiver operating characteristic (roc) curves were used to determine the test’s overall predictive accuracy (area under the curve) and to identify optimal posterior corneal elevation cut off points to maximize sensitivity and specificity in discriminating keratoconus and subclinical keratoconus from normal corneas. similar analysis was done using orbscan ii, in the central 5mm zone. results: in galilei mean posterior corneal elevation in the 3 mm zone was statistically higher in keratoconus (50.6 ± 34.5 microns), and subclinical keratoconus (12.8 ± 4.3 microns) versus normal corneas (9.9 ± 4.5 microns). roc curve analyses showed high overall predictive accuracy of posterior elevation for both keratoconus and subclinical keratoconus (area under the curve 98% and 71%, respectively). optimal cutoff points were 18.5 microns for keratoconus and 11.5 microns for subclinical keratoconus. these values were associated with sensitivity and specificity of 92% and 95% respectively for keratoconus, and 64.3% and 75.7% for subclinical keratoconus. in orbscan ii mean posterior corneal elevation in the 5 mm zone was 106.8 ± 43.7 microns in keratoconus, 36.6 ±22.8 microns in subclinical keratoconus and 25 ± 9.2 microns in normal corneas. optimal cutoff points were 50 microns for keratoconus and 33.5 microns for subclinical keratoconus. these values were associated with sensitivity and specificity of 89.1% and 98.6% respectively for keratoconus, and 50% and 85% for subclinical keratoconus.                     conclusion: posterior corneal elevation very effectively discriminates keratoconus from normal corneas. its accuracy is lower in subclinical keratoconus and thus data concerning posterior elevation should not be used alone to stratify patients with this condition.