Influence of central corneal thickness on measured intraocular pressure differentials: Nidek RKT-7700, Topcon CT-80 NCTs and Goldmann Tonometer

Purpose We sought to compare the intraocular pressure (IOP) measured by RKT-7700 and CT-80 noncontact tonometers (NCTs) with that measured by Goldmann applanation tonometry (GAT). We also examined the influence of central corneal thickness (CCT) on the agreement between both NCTs and GAT in a sub-population of healthy, young normals. Methods Triplicate IOP and CCT measurements were obtained twice from one randomized eye of 49 subjects (28 males and 21 females) aged 22.2 +/- 1.3 (mean +/- S.D.) years. Goldmann tonometry was performed subsequent to assessment with the RKT-7700 and CT-80, to negate the ocular massage effect, followed by ultrasound pachymetry. The results from each method were compared and assessed for repeatability and between-observer reproducibility. Analysis was performed to determine the correlation between the differences in IOP measurements and corneal thicknesses. Results The mean differences (+/- S.D.) in sessions 1 and 2 respectively between RKT-7700 and GAT (2.6 +/- 2.0 and 2.7 +/- 1.4 mmHg), between CT-80 and GAT (2.8 +/- 2.0 and 3.2 +/- 1.3 mmHg) were statistically significant (p < 0.0001). The repeatability coefficients in sessions 1 and 2 respectively were; +/- 1.2 and +/- 1.0 mmHg (GAT), +/- 1.3 and +/- 1.6 mmHg (CT-80), +/- 2.3 and +/- 1.7 mmHg (RKT-7700) and inter-observer reproducibility was; +/- 1.9 (RKT-7700), +/- 2.3 (GAT) and +/- 2.6 mmHg (CT-80). Between the corneal thickness and the differences in GAT-measured and NCT-measured IOPs, there was a significant negative correlation and a trend for a larger difference in thicker corneas. Conclusion Neither the RKT-7700 nor the CT-80 can be used interchangeably with the Goldmann tonometer, although all three tonometers give repeatable measurements of IOP, in this sub-population. IOP differences between GAT and the NCTs significantly correlated with CCT, with the possibility of even greater differences in thicker corneas.