On the reliability (repeatability) of single pachymetry measures taken with specular microscope-based assessments of the human corneal endothelium.

Abstract

To assess agreement between 3 measures of central corneal thickness (CCT) taken over a few minutes from nominally normal eyes with a non-contact specular microscope. 100 eyes from 100 healthy adults (with an average age of 22 y) were assessed using the Topcon 3000 P instrument to obtain a high quality image of the endothelium and pachymetry. The group mean CCT values from the 1st, 2nd and 3rd measures were 0.519, 0.520 and 0.520 mm, but the sets of values could differ by between - 0.020 mm and + 0.029 mm, i.e. between - 4.0 and + 5.6% of the average values. Paired comparisons (e.g. 2nd vs. 1st set) indicated limits of agreement (LoA) to be between - 0.020 and + 0.020 mm of the averaged value. Across the 3 measures, the averaged SD was 0.005 mm to give an estimate of the intra-subject variability (as the coefficient of variation, COV) of 0.91% (range 0 to 2.3%). The variability in the pachymetry measures was not predictably related to the averaged values of CCT (r = 0.022) or self-reported refractive error of the subjects (r = 0.012). Some repeat pachymetry assessments obviously included the same region of the endothelium. Single pachymetry measures with a non-contact specular microscope are only likely to be able to generate CCT estimates within +/- 4% of the expected average values. This repeatability is comparable to single image estimates of endothelial cell density and therefore acceptable in most cases.