Clinical evaluation of five portable tonometers

Intraocular pressure (IOP) measurements obtained with five commercially available portable tonometers (Keeler Pulsair 2000, Tomey Pro-Ton, Tono-Pen XL, Perkins, and Shiotz) were compared to IOPs measured with a Goldmann tonometer in order to predict whether instrument accuracy justified substitution of a portable instrument for the Goldmann in all or specific circumstances.

A total of 31 patients (59 eyes) were assessed. The order of techniques was random except that in each case Goldmann tonometry was performed first and Shiotz tonometry was performed last. Examiners were masked to all other IOP measurements. Correlation and regression analyses were used to predict Goldmann IOP from the other portable tonometer readings. In addition, the difference in pressure readings was compared to mean pressure readings.

Comparisons of means, correlations, regression equations, and box plots of difference scores were performed to determine which techniques were more accurate. The results varied slightly depending on the method used. All IOPs obtained with portable tonometers showed significant correlation and regression equations compared to the Goldmann IOPs. The correlation ranged from (r = 0.75) for the Keeler Pulsair to (r = 0.66) for the Schiotz. Three tonometers (Tono-Pen, Schiotz, and the Pro-Ton) showed a small significant underestimation of the Goldmann IOPs.

All portable tonometers were able to accurately predict Goldmann IOPs. While three of the tonometers were found to underestimate Goldmann IOP on average, these results can be corrected by either adding the underestimation or by using the best fitting regression equation.