Abstract
Purpose :
The study aimed to determine the pachymetry error of Pentacam’s tomographer, with a standard resolution of 25 images per second, at the apex, pupil center, and thinnest point of the cornea in a population of young individuals with keratoconus.
Methods :
We did Pentacam’s pachymetry in 104 right eyes and 104 left eyes, of distinct individuals, with keratoconus in at least one eye. Their ages varied from 12 to 25-year-old. Each eye was measured by the same operator, two times in sequence, separated by an interval of 10 to 20 minutes, using a standard resolution of 25 images per second. Pachymetry error (repeatability coefficient) was calculated from the differences between the first and second measurements of the same eye using a variation of the Bland-Altman approach. Data of interest were the thickness of the apex, pupillary center, and thinnest point of the cornea. Only scan results with the quality factor (QS) above 95% were used. The formula for the repeatability coefficient was RC = 2.77 sw, where sw is the within-subject standard deviation derived from the square root of the residual mean square of an analysis of variance with the subject as a factor. The formula for the standard error of the CR was SE = 2.42 sw / N1/2 estimated the sample size (N) with an expectative of a confidence interval around ± 0.24 sd.
Results :
There were no significant differences among pachymetry errors at the apex, pupillary center, and thinnest point of the cornea. The 95% CI of the error was 13 to 16 µm.
Conclusions :
To simplify matters, we propose to use the upper limit of 95% CI, say 16 µm, as the pachymetry error of Pentacam when using the standard resolution of 25 images per second. Any discrepancy among measurements below this limit must be interpreted as insignificant, clinically, and statistically.
This is a 2020 ARVO Annual Meeting abstract.