Agreement among 3 Methods to Measure Corneal Thickness: Ultrasound Pachymetry, Orbscan II, and Visante Anterior Segment Optical Coherence Tomography Emmy Y. M. Li, MBBS, Shaheeda Mohamed, MRCSEd, Christopher K. S. Leung, MD, MBChB, Srinivas K. Rao, FRCS, Arthur C. K. Cheng, MRCS, FCOphth(HK), Carol Y. L. Cheung, PhD, Dennis S. C. Lam, MD, FRCOphth Purpose: To assess the agreement of central and paracentral corneal thickness measurements between ultrasound pachymetry (USP), Orbscan II, and Visante anterior segment optical coherence tomography (ASOCT). Design: Observational cross-sectional study. Participants: Seventy eyes of 70 subjects. Methods: Each subject underwent Orbscan II (using an acoustic equivalent correction factor of 0.89), ASOCT, and USP examination. Bland–Altman plots were used to evaluate agreement between instruments. Main Outcome Measures: Central and paracentral corneal thickness measurements by the 3 methods and agreement, as evaluated by 95% limits of agreement (LOA). Results: The mean measurements of average central corneal thickness by USP, Orbscan II, and ASOCT were 553.530.26 m, 553.2225.47 m, and 538.7926.22 m, respectively. There was high correlation between instruments: USP with ASOCT (r = 0.936, P0.001), USP with Orbscan II (r = 0.900, P0.001) for central corneal thickness measurements, and Orbscan II with ASOCT for average paracentral 2- to 5-mm measurements (r = 0.947, P0.001). The mean differences (and upper/lower LOA) for central corneal thickness measurements were 0.3113.34 m (26.44/-25.83) between USP and Orbscan II, 14.7410.84 m (36.0/ -6.51) between USP and ASOCT, and 14.449.14 m (32.36/-3.48) between Orbscan II and ASOCT. The average mean difference (and upper/lower LOA) between Orbscan II and ASOCT for paracentral 2- to 5-mm corneal thickness measurements was 10.358.67 m (27.35/-6.65). Conclusion: Anterior segment optical coherence tomography underestimated corneal thickness compared with that measured with USP. Anterior segment optical coherence tomography had better agreement with the gold standard USP, as compared with Orbscan II. However, important discrepancies among instruments exist. Clinicians should be aware that corneal thickness measurements are influenced by the method of measurement and that, although highly correlated, these instruments should not be used interchangeably for the assessment of corneal thickness. Ophthalmology 2007;114:1842–1847 © 2007 by the American Academy of Ophthalmology. Corneal thickness measurements are clinically important. Variation in corneal thickness affects the accuracy of ap- planation tonometry. A meta-analysis by Doughty and Za- man showed that a 10% change in central corneal thickness may result in an approximately 3.4-mmHg change in in- traocular pressure. 1 Accuracy of corneal thickness measure- ment is also important in considering eligibility for refrac- tive surgery, and the amount of correction that can safely be performed. Underestimation of corneal thickness may lead to eligible patients being excluded for primary or enhance- ment refractive procedures. Overestimation may lead to overablation and inadvertent thinning of the stroma, thereby increasing the risk of iatrogenic keratectasia. Hence, studies assessing corneal thickness measurements by various instru- ments are of high clinical relevance. 2–9 Recently, anterior segment optical coherence tomography (ASOCT) has been developed to image the anterior segment using noninvasive low-coherence interferometry 10 and provides pachymetry mapping of the cornea. Pachymetric mapping offers the Originally received: September 14, 2006. Final revision: February 14, 2007. Accepted: February 15, 2007. Available online: May 15, 2007. Manuscript no. 2006-1038. From the Department of Ophthalmology and Visual Sciences, Chinese University of Hong Kong, Hong Kong, China. The authors have no proprietary interest in the development or marketing of any product mentioned in the article, and the study receives no financial support. Drs Li and Mohamed share first authorship. Correspondence and reprint requests to Christopher K. S. Leung, MD, MB, ChB, Department of Ophthalmology and Visual Sciences, Chinese University of Hong Kong, Hong Kong, China. E-mail: tlims00@hotmail.com. 1842 © 2007 by the American Academy of Ophthalmology ISSN 0161-6420/07/$–see front matter Published by Elsevier Inc. doi:10.1016/j.ophtha.2007.02.017