Sensitivity and specificity of posterior corneal elevation measured by Pentacam in discriminating keratoconus/subclinical keratoconus

Purpose: To estimate the sensitivity and specificity of posterior elevation in discriminating keratoconus and subclinical keratoconus from normal corneas. Design: Prospective case-control study. Participants: Seventy-five patients with keratoconus, 25 with subclinical keratoconus, and 64 refractive surgery candidates with normal corneas. Methods: In one eye of each patient, posterior corneal elevation was measured in the central 5 mm using the Pentacam rotating Scheimpflug camera (Oculus, Wetzlar, Germany). Posterior corneal elevation in keratoconus and subdinical keratoconus were compared with that in normal corneas in separate analyses. 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 cutoff points to maximize sensitivity and specificity in discriminating keratoconus and subdinical keratoconus from normal corneas. Logistic regression was used to support cutoff points identified through ROC curve analysis, and to check for model validity; model goodness-of-fit was estimated using r(2), and its internal validation was by bootstrapping analysis. Main Outcome Measures: Posterior corneal elevation in keratoconus, subdinical keratoconus, and normal corneas. Results: Mean posterior corneal elevation was statistically higher in keratoconus (100.7 +/- 49.2 mu m; P<0.001), and subdinical keratoconus (39.9 +/- 15.0 mu m; P = 0.01) versus normal corneas (19.8 +/- 6.37 mu m). ROC curve analyses showed high overall predictive accuracy of posterior elevation for both keratoconus and subdinical keratoconus (area under the curve 0.99 and 0.93, respectively). Optimal cutoff points were 35 Am for keratoconus and 29 mu m for subdinical keratoconus. These values were associated with sensitivity and specificity of 97.3% and 96.9%, respectively, for keratoconus, and 68% and 90.8% for subdinical keratoconus. Similar cutoff points were obtained with logistic regression analysis (38 mu m for keratoconus and 32 Am for subdinical keratoconus). The models showed good fit to the data, including after internal validation. Conclusions: Posterior corneal elevation very effectively discriminates keratoconus from normal corneas. Its efficacy is lower for subdinical keratoconus, and thus data concerning posterior elevation should not be used alone to stratify patients with this condition.