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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 [mu]m for optical pachymetry, 542 /- 33 [mu]m for ultrasound pachymetry, and 596 /- 40 [mu]m for Orbscan pachymetry. For day-to-day comparisons, optical pachymetry showed the poorest repeatability with 95% LoA of -61 to 32 [mu]m. Ultrasound pachymetry showed better repeatability with 95% LoA of -22 to 24 [mu]m. The Orbscan showed the best repeatability centrally with 95% LoA of -10 to 17 [mu]m. 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.

(C) 2000 Lippincott Williams & Wilkins, Inc.