[A new site-independent tonometer (ProTon) in comparison with the Goldmann applanation tonometer]

A new position-independent and portable Mackay-Marg-tonometer in miniature, the so-called ProTon (PT), was compared with reference to its clinical usefulness with the Goldmann applanation tonometer (GAT), being a commonly accepted reference.

The intraocular pressure (IOP) in 164 eyes of 85 adults (patients and volunteers without ocular disease) was measured using the sequence GAT, PT and again GAT--each three times in a row-by three different investigators in a randomized scheme. In an attempt to detect any systemic influence, the difference obtained by subtracting the value measured using PT from the value attained using GAT was compared to the central corneal thickness and radius. Moreover, the central IOP readings were compared with the peripheral IOP in 60 eyes of 30 volunteers measured by PT, to detect any possible influence of the locality of measurement. Dependence of measurement on individual influences was examined by comparing PT-values obtained by each investigator.

Linear regression analysis between PT (y-axis) and GAT (x-axis) for an IOP range of 0 to 62 mm Hg resulted in the following equation: PT = 1.036 x GAT -2.00 with a PT value standard deviation of 1.65 mm Hg (r = 0.988; p < 0.0001). Reproducibility of each of the three PT-values (mean pair difference of 1.1 mm Hg) corresponded to the analogous first two GAT-values (differences 1.0 up to 1.2 mm Hg). The mean IOP-value of 1) the GAT measurements (1 to 3) was 18.35 mm Hg, 2) of the last three GAT measurements, 17.15 mm Hg and 3) of all PT-measurements, 16.40 mm Hg. No statistical difference was found between the IOP measured centrally and peripherally. A clinically nonsignificant dependency for PT-readings on the examiner was verified.

Due to the high reproducibility, independency on examiner and acceptable correspondence of PT with GAT values, the PT appears to be a promising new development for use in measuring intraocular pressure. It will be necessary to modify the calibration of PT since IOP measured systematically about 2 mm Hg too low.