Pachymetric Thresholds for Keratoconus Progression Based on MS-39 Measurement Precision.

PURPOSE: To evaluate the precision of pachymetry determined using the MS-39 device across different levels of keratoconus (KC) and to establish thresholds for detecting disease progression. METHODS: This study examined 27, 29, 34 and 41 eyes with forme fruste keratoconus (FFKC), mild KC, moderate KC and severe KC, respectively. To assess repeatability, single-measurement and three-measurement reproducibility, three consecutive measurements were obtained with the MS-39 by two well-trained operators. The within-subject standard deviation (Sw), test-retest variability (TRT), coefficient of variation (CoV) and intraclass correlation coefficient (ICC) were employed for statistical analysis. RESULTS: The MS-39 demonstrated high repeatability and reproducibility in measuring corneal thickness in KC patients, with all ICC values exceeding 0.90. Based on precision analyses, the following critical values for the thinnest corneal thickness are recommended: for FFKC, mild, moderate and severe KC stages, thresholds should be set at 4.43, 6.58, 8.34 and 16.71 µm, respectively, when using triple-averaged measurements. Conversely, for single measurements, the corresponding thresholds are 6.68, 10.89, 10.20 and 23.31 µm. Exceeding these values should prompt careful re-evaluation of measurement error or progression of the disease. CONCLUSIONS: The MS-39 had high repeatability and reproducibility across different levels of KC severity. Given the corneal irregularity in KC, averaging three measurements may improve reliability and reduce errors in clinical decision-making. Moreover, grading KC patients accurately and applying different thresholds is an effective approach.