Purpose: To evaluate the intra- and interobserver variability of the Ocuton S tonometer, its correlation with Goldmann tonometry, the reliability of self-tonometry and the safety of the instrument. Methods: Thirty-five healthy subjects and 45 patients with primary open-angle glaucoma (POAG), aged from 38 to 80 years (mean age: 64.6 +/- 12.2 years), underwent tonometry with the Ocuton S tonometer in one eye chosen at random. The intra- and interobserver variability between two operators (kappa coefficient), the Ocuton S/Goldmann correlation and the reliability of self-tonometry were evaluated by performing two tonometries on each patient in subgroups. Each tonometry was considered as the mean of three consecutive measurements. Central ultrasonic pachymetry, keratometry and corneal biomicroscopy were also evaluated. Results: The intra- and interobserver variability ranged from 0.38 to 0.66. The difference between the means of intraocular pressure (IOP) with the Ocuton S (24.4 +/- 4.7 mmHg) and the Goldmann tonometer (18.1 +/- 4.7 mmHg) was statistically significant (p < 0.0001). Linear regression analysis revealed a good Ocuton S/Goldmann correspondence (r = 0.88, p = 0.0001). However, IOP values detected with the Ocuton were consistently overestimated, compared to those detected with the Goldmann tonometer. The correlation between corneal thickness and IOP was statistically significant both for the Goldmann (r = 0.510, p = 0.021) and for the Ocuton S tonometer (r = 0.520, p = 0.019). No correlation was found between keratometry and IOP. The mean measurement obtained by self-tonometry (21.9 +/- 3.6 mmHg) showed no statistically significant difference when compared to the mean measurement obtained by an expert operator (21.3 +/- 3.4 mmHg). Conclusion: The Ocuton S tonometer is a safe instrument that can be used easily by the patient. However, in comparison to the Goldmann tonometer, it overestimates IOP and requires further technical and methodological refinements in order to ensure greater reliability.
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