Comparison of the Tono-Pen and Goldmann tonometer for measuring intraocular pressure in patients with glaucoma

Aim: To estimate agreement between measurement of intraocular pressure (IOP) by the Tono-Pen and by the Goldmann tonometer. The hypothesis tested was that the mean difference in measurements would be <-2 mmHg. Methods: A prospective, single-centre, comparison study was carried out. The Tono-Pen was compared to the Goldmann tonometer in 138 patients systematically selected from two glaucoma clinics and among selected patients known to have high pressures (n = 22). For both groups the investigators were masked to the value of the Goldmann readings, the instruments were used in random order, and duplicated readings were taken with both tonometers. Results: In the systematically selected group, for right eyes, the mean difference in measurement between the Tono-Pen and Goldmann tonometer was -0.41 mmHg (SD: 2.59). The 95% limits of agreement were -5.5 to 4.7 mmHg, and the intraclass correlation coefficient (ICC) was 0.90. Results for left eyes were very similar. For patients with high pressures (greater than or equal to25 mmHg), the mean difference between instruments (Tono-Pen reading minus Goldmann reading): was -4.2 mmHg (P = 0.0004), SD: 4.6, 95% limits of agreement: -13.2 to 4.8 mmHg and ICC = 0.76. Combining the analysis for both groups, the Tono-Pen significantly underestimated the IOP when the pressure was >20 mmHg. Conclusions: The Tono-Pen cannot replace the Goldmann tonometer in the sense that it will give the same readings of IOP. The accuracy of the Tono-Pen is increased, if at least two measurements are taken per eye and then averaged.