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 (> or =25 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.