Purpose: To determine if there is a systematic difference in central corneal thickness (CCT) measured using anterior segment optical coherence tomography (AS-OCT) as compared with ultrasound pachymetry. Design: Prospective observational study. Methods: Consecutive subjects with clinically normal corneas underwent CCT measurement by both ultrasound and AS-OCT while participating in a population-based study in Ghana, West Africa. One eye of each subject was randomly selected for analysis. Two measurements were taken and averaged. Agreement and interobserver variability were also analyzed. Results: One hundred and fifty-five subjects of African ethnicity and average age 57 years (standard deviation [SD] 12; range, 40 to 98 years) were included. Measurements by AS-OCT and US were taken a mean of 15 days (maximum, six weeks) apart. The mean (SD) [range] US CCT was 525.3 μm (33.5) [422, 653] and 499.0 μm (32.0) [428, 613] with AS-OCT. Measurements by the two modalities were strongly correlated (r2 = 0.82; P < .001), and a significant difference was observed between mean US and AS-OCT CCT (SD) [range] of 26.3 μm (14.2) [-63, 12] (P < .001). The width of the limits of agreement was 28 μm, about 6% of the average pachymetry reading. In 50 eyes randomly remeasured with OCT by a second observer, the intraclass correlation coefficient was 0.91. There was a small but significant systematic difference between observers (mean 6.9 μm, SD 10.9 μm), or 1.4% (P < .001), increasing the difference noted above. Conclusion: There is a reproducible systematic difference between CCT measurements taken with ultrasound and OCT. It is important to note in clinical practice, that measurements acquired by these two modalities are not directly interchangeable.
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