The repeatability of corneal thickness measures

Purpose. To compare the repeatability of three measures of corneal thickness: Orbscan Slitscan pachymetry, ultrasound pachymetry, and optical pachymetry. Methods. Twenty normal subjects were tested on three occasions. Two occurred on the same day and the third was on a different day at approximately the same time of day as one of the first two visits. Central corneal thickness of the right eye was measured with a Haag-Streit optical pachymeter, a Humphrey Model 855 ultrasound pachymeter, and the: Orbscan system. Day-to-day and same-day repeatability was assessed by calculating the difference between the values from two visits and determining the mean difference, the SD, and the 95% Limits of agreement (LoA) (LoA = mean +/- 1.96 SD). Results. Mean (+/-SD) central corneal thickness as measured by each instrument was as follows: 539 +/- 33 mum for optical pachymetry, 542 +/- 33 mum for ultrasound pachymetry, and 596 +/- 30 mum for Orbscan pachymetry. For day-to-day comparisons, optical pachymetry showed the poorest repeatability with 95% LoA of -61 to +32 mum. Ultrasound pachymetry showed better repeatability with 95% LoA of -22 to +24 mum. The Orbscan showed the best repeatability centrally with 95% LoA of -10 to +17 mum. Peripheral Orbscan pachymetry was less repeatable than that measured centrally but still more repeatable than central optical pachymetry. Similar results were found with same-day comparisons. Conclusion. The Orbscan system is the most repeatable technique for measuring corneal thickness but shows a significant bias toward greater corneal thickness measures than both ultrasound and optical pachymetry.