ARTICLE Repeatability of Central Corneal Thickness Measures by Orbscan Pachymetry for Right and Left Eyes Sven Jonuscheit, M.Sc.Optom., Ph.D., and Michael J. Doughty, Ph.D. Objectives: To assess the repeatability of Orbscan measures on right versus left eyes. Methods: Three Orbscan measures were taken from the right eye and then the left eye by the same operator on 50 healthy myopic subjects (average right eye 4.3 DS) aged 19 to 58 years, half of whom were successful long-term soft contact lens (SCL) wearers. The central point thickness measurement was compared for the 3 scans for each eye, the variability calculated based on the coefficient of variation (COV) and also paired comparisons with limits of agreement. No acoustic factor was applied. Results: Single central point thickness values ranged from 0.483 to 0.663 mm, with mean values of 0.579 mm for both eyes, with no statistically significant difference between spectacle wearers and SCL wearers (P0.387). From 3 repeat scans, the net difference was 0.001 mm between third and first scans of right eyes and 0.001 mm for left eyes (P0.5). Overall, the COV was close to 1%, being marginally higher (0.955% and 1.176%) in spectacle wearers than SCL wearers (0.922% and 0.836%; P, NS). However, an overall assessment of the COV data indicates that the values were inversely related to absolute thickness values, being slightly higher for thinner versus thicker corneas (P0.01). The limits of agreement (95% confidence interval) between repeat sets of measures were 0.018 mm for third versus first scans of either eye or 0.023 mm for first scans of right versus left eyes. Conclusions: The results confirm excellent repeatability of Orbscan pachymetry measures in both spectacle and contact lens wearers. For the adult subjects assessed, there was no evidence of a right/left eye difference and/or a progressive change in measured thickness with time. The high similarity between right and left eye measures should be taken into account if data from both eyes are being used either in contralateral eye compari- sons or in comparative studies of different pachymeters. Key Words: Pachymetry—Central corneal thickness—Human—Orbscan. (Eye & Contact Lens 2009;1: 2025) T he clinical measurement of central corneal thickness (CCT) can be made with a variety of optical or ultrasound-based methods. 1 Older studies using optical (slitlamp-based) measures con- sidered that “The limits of normal variation (in CCT) were 0.46 and 0.67 mm.” 2 A meta-analysis, with calculation of a 95% confidence interval from 230 sets of published data, indicated that the normal CCT value could be considered to be between 0.473 and 0.597 mm. 1 One of the more recently introduced instruments is the Orb- scan. 3 With the introduction of this, and several other newer instruments, the main interest seems to have been in assessments of the accuracy and repeatability of pachymetry. An opinion has been given that Orbscan is “more reliable than simpler optical point pachymetry because it reduces or eliminates several biases and sources of error.” 4 Although having a completely automated scanning sequence and image acquisition (which includes a system for monitoring the actual position of the eye over the relatively short period of time required to obtain a set of images), 5 it has been commented that an “inherent weakness (in the use of Orbscan) in obtaining precise corneal thickness measurements is that it re- quires a patient to fixate for 1.0 to 1.5 sec.” 6 Regardless of the absolute accuracy of Orbscan measures on a living human cornea, opinions have been given that 3 repeat scans should be done “to ensure repeatability and accuracy.” 7 Indeed, a number of other investigators have opted to take 3 scans when using Orbscan, 8 –14 although others have reported using just 2 scans, 6,15–20 or even taking 5 scans. 5,21,22 In all cases, repeat scans have been used to obtain an averaged value for further analyses. In these various studies and reports, however, no obvious distinction seems to have been made between any patient-depen- dent factors that might determine the repeatability of the scans. These could include reliability and/or repeatability of fixation, devel- opment of fatigue (tiring), or a progressive improvement with learning (assuming in all cases that the patient has no binocular vision or other ocular imbalance problems). Similarly, repeat measures could be affected by either corneal drying (leading to thinning) or reflex tearing (leading to at least an apparent increase in thickness). All could contribute to variability of repeat measures. It has become commonplace for many of the comparative pachymetry studies using Orbscan for data from both eyes to be used. 6 –9,13,18,20,21,23– 41 A related issue to patient-dependent factors is whether the sources of error for each eye should be considered as different or the same. If they were different, then each eye could be considered as an independent measure of corneal thickness. However, if the repeatability of measures from both eyes are so similar (due to both their thickness essentially being the same and the net effect of any errors in measurement being identical), then the measures from the 2 eyes should be considered as essentially interdependent. If this can be demonstrated, then it can be argued that both eyes should not be used in comparisons between different pachymeters. The present studies were undertaken to assess repeatability of Orbscan measures of CCT, with special attention being given to the timing of the scans, the magnitude of the variability in repeat From the Department of Vision Sciences, Glasgow-Caledonian Univer- sity, Glasgow, Scotland, United Kingdom. The authors have no proprietary or financial interests in any of the equipment or techniques used in the present study. Address correspondence and reprint request to Michael J. Doughty, Ph.D., Department of Vision Sciences, Glasgow-Caledonian University, Cowcaddens Road, Glasgow G4 OBA, Scotland, United Kingdom; e-mail: m.doughty@gcal.ac.uk Accepted October 27, 2008. DOI: 10.1097/ICL.0b013e318192d89a 20 Eye & Contact Lens • Volume 35, Number 1, January 2009