[Clinical calibration and in vitro evaluation of the validity of 2 microprocessor controlled tonometers]

In a randomised clinical study two new microprocessor-controlled tonometers working according to the Mackay-Marg principle (Tono-Pen and ProTon, two samples of each) and two established applanation tonometers [hand-held applanation tonometer, Draeger (HAT) and Goldmann tonometer] were compared with respect to accuracy, precision, examiner dependence and internal measurement differences (Tono-Pen I vs II, ProTon I vs II). The four tonometers were used by two examiners six times on each of 193 eyes of 100 patients of the university eye department. The same measuring procedure was performed on four cadaver eyes in situ, using a manometer system. The correlation coefficient between Tono-Pen and HAT and between ProTon and HAT was 0.9, with a high standard deviation, about +/- 3.1 mmHg. HAT gave the best results in repetitive measurements in the same eye (precision: r = 0.92, SD = +/- 1.41 mmHg). In applanation tonometers but not in Mackay-Marg tonometers measurement values depended significantly on the physician. There was a significantly higher standard deviation between Tono-Pen I and II than between ProTon I and II. Concerning in vitro calibration series on human bulbs (manometry), accuracy was high with each tonometer. The regression line of HAT is similar to that of ProTon. Tono-Pen overestimates the manometrically determined intraocular pressure. Because of their lack of precision the two new tonometers are not suitable for clinical use.