The mean ± SD uncorrected Cirrus optic disc area was 1.86 ± 0.46 mm
2, which was not significantly different from corrected Cirrus disc area measurements (1.87 ± 0.46 mm
2,
P = 0.655, Wilcoxon's signed rank test). The K-corrected HRT disc area was on average larger than the AL-corrected HRT disc area (2.02 ± 0.50 vs. 1.89 ± 0.49 mm
2, respectively;
P < 0.001, Wilcoxon's signed rank test). The difference between the AL-corrected HRT and uncorrected or corrected Cirrus disc area measurements were not significant (
P > 0.481). However, the distribution or spread of the differences between the HRT and Cirrus SD-OCT disc areas did not significantly decrease after correction of the HRT disc area for magnification with Bennett's method (
P = 0.262, nonparametric resampling test).
Table 2 compares the main uncorrected HRT and Cirrus parameters. Magnification correction could be performed only for the disc area with HRT images because of technical reasons.
Table 3 demonstrates the pairwise ICCs between the K-corrected and AL-corrected HRT disc areas and uncorrected and corrected Cirrus disc areas (
Figs. 3A,
3B). Bland-Altman plot showed a slight negative trend for the difference of K-corrected HRT disc area and corrected Cirrus disc area with increasing disc size, which was not statistically significant (β = −0.052,
P = 0.454;
Fig. 4A). The K-corrected HRT disc area measurements were consistently higher than corrected Cirrus measurements at all axial lengths (
Fig. 5). The results of univariate regression analyses demonstrated that axial length (β = 0.063;
P = 0.346) and disc area (β = 0.040;
P = 0.454) did not have any influence on the difference between the K-corrected HRT and corrected Cirrus disc areas. The effect of spherical equivalent was of borderline significance (β = −0.025;
P = 0.058), whereas the average keratometry measurement was significantly associated with the disc area difference (β = 0.044;
P = 0.008). Results of multivariate analyses, including corrected Cirrus disc area, AL (or spherical equivalent), keratometry readings, age, or glaucoma severity, did not change the results (
P = 0.007 vs. 0.014 for K-readings in analyses including AL vs. spherical equivalent, respectively). The results were fairly similar for the difference between K-corrected and AL-corrected HRT disc area (
P = 0.087, 0.273, and 0.454 for AL, spherical equivalent, and disc area, respectively, and
P = 0.005 for K-reading). The difference between uncorrected and corrected Cirrus disc areas did not vary as a function of corrected disc area, or K-readings, when adjusted for axial length (data not shown).