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Editors Selection IGR 24-1

Intraocular pressure: GAT calibration

Comment by Arthur Sit on:

19914 Calibration errors of Goldmann tonometers in a tertiary eye care centre, Chuo JY; Mikelberg FS, Canadian Journal of Ophthalmology, 2007; 42: 712-714

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Goldmann applanation tonometry (GAT) was first described over fifty years ago. Since then a multitude of new instruments have been invented and adopted for measuring intraocular pressure (IOP), but GAT remains the reference standard for all other tonometers. In this context, it is surprising how often calibration of this mechanical device is neglected with the assumption that it will maintain accuracy for extended periods of time Chuo et al. (1247) evaluated the prevalence and magnitude of Goldmann tonometer calibration errors in a tertiary eye care center. Using standard calibration weight bars for Haag-Streit tonometers, they measured calibration error at three different pressure levels (0, 20, and 60 mmHg) in all 25 tonometers of their clinic. At the most clinically significant calibration level of 20 mmHg, they found that 32% of tonometers were outside of the manufacturer's specified tolerance of ± 0.5mmHg and 24% of tonometers had errors ≥ 1.5 mmHg. Although not specifically commented upon, 12% of tonometers had errors greater than 4 mmHg.

At the most clinically significant calibration level of 20 mmHg, they found that 32% of tonometers were outside of the manufacturer's specified tolerance

The rate of calibration errors found by Chuo et al. Was greater than the report of Wessels et al. (Arch Ophthalmol 1990; 108: 1709-1712) but markedly less than the report of Sandhu et al. (J Glaucoma 2005; 14: 215-218). The reasons for the differences are unclear and no other data on the accuracy of Goldmann tonometers in real-world use are available in the literature.

The limitations of this study are its small size and cross-sectional design (which does not allow assessment of time from last calibra-tion). As well, the authors did not specify the model numbers of the devices used. This would be important since some designs for Goldmann tonometers generate the applanation force using a spring while others use a leveraged weight. Whether one design or the other is more robust is unclear but further investigation is needed. Nevertheless, this study highlights the need to better understand the rate and source of Goldmann tonometer calibration errors as IOP remains a critical parameter in glaucoma management.

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