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

Abstract

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. 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. The mean differences (±S.D.) in sessions 1 and 2 respectively between RKT-7700 and GAT (2.6±2.0 and 2.7±1.4mmHg), between CT-80 and GAT (2.8±2.0 and 3.2±1.3mmHg) were statistically significant (p<0.0001). The repeatability coefficients in sessions 1 and 2 respectively were; ±1.2 and ±1.0mmHg (GAT), ±1.3 and±1.6mmHg (CT-80), ±2.3 and ±1.7mmHg (RKT-7700) and inter-observer reproducibility was; ±1.9 (RKT-7700), ±2.3 (GAT) and ±2.6mmHg (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. 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.