Disposable devices for measuring intraocular pressure: a clinical study to assess their accuracy

We conducted a study to compare the accuracy of two commercially available disposable tonometry devices (the acrylic biprism, Tonosafe and the silicone shield, Tonoshield) with the standard Goldmann applanation tonometer (GAT).

A total of 80 eyes of 40 patients were randomly assigned to two groups. Group A (40 eyes) had the intraocular pressure IOP measured using GAT and the acrylic biprism (Tonosafe), while Group B (40 eyes) had IOP measured using GAT and the disposable silicone shield (Tonoshield). IOP was measured using the GAT both before and after the disposable device and average of the two standard Goldmann readings was compared with the IOP measured using the disposable devices.

The mean difference in IOP in Group A was 0.29 mmHg (SD 0.54) (P=0.0018) while in Group B the mean difference in IOP was 2.09 mmHg (SD 1.23) (P<0.0001). The Altman-Bland method was used to assess the agreement between two methods of clinical measurement. This shows a positive bias of 0.29 (95% confidence interval (CI) 0.11-0.46) when using the disposable acrylic biprism and a positive bias of 2.09 (95% CI 1.69-2.48) with the silicone shield.

The IOP recorded using the disposable acrylic biprism (Tonosafe) was in close agreement with the standard GAT. However, IOP readings obtained with the silicone shield (Tonoshield) tend to be higher than with the standard GAT. These facts should be kept in mind when recording IOP in clinical situations.