Comparability and repeatability of pachymetry in keratoconus using four noncontact techniques

PURPOSE: To compare and determine the repeatability of central corneal thickness (CCT) measurements using four noncontact pachymetry instruments in eyes with keratoconus. MATERIALS AND METHODS: The CCT of consecutive patients with keratoconus was measured during a single visit using the swept source optical coherence tomography (SS-OCT, Casia SS-1000°CT, Tomey, Nagoya, Japan), a rotating Scheimpflug camera system (Pentacam, Oculus Optikgerate GmbH, Wetzlar, Germany), scanning slit topographer (Orbscan IIz topography, Baush and Lomb Surgical Inc., San Dimas, CA, USA), and a hand-held spectral domain OCT (HHSD-OCT, Bioptigen Inc., Durham, North Carolina, USA). Test-retest variability, correlation between measurements and interdevice agreement were analyzed. RESULTS: Fifty eyes of 25 participants were analyzed in this study. All measurement methods correlated well with each other (r > 0.9, P < 0.001). Mean ± standard deviation CCT measured by HHSD-OCT, Orbscan IIz, SS-OCT, and Pentacam was 462 ± 41 μm, 458 ± 41 μm, 454 ± 40 μm, and 447 ± 42 μm, respectively. While the HHSD-OCT over-estimated the CCT (P < 0.001), there was a good correlation between the measurements obtained from the other three devices. However, the numerical difference was high and this trend was seen in all the paired comparisons. CONCLUSIONS: Though the measurements by different devices correlated well, the numerical agreement may be inadequate for their interchangeable use in clinical practice.