Calibration errors of Goldmann tonometers in a tertiary eye care centre

To evaluate the calibration errors of all Goldmann applanation tonometers used clinically in the Vancouver General Hospital/University of British Columbia (VGH/UBC) Eye Care Centre.

All Goldmann tonometers used for patient care in the VGH/UBC Eye Care Centre were individually assessed for calibration errors. The tonometers were checked according to the Haag-Streit method using a standard calibration weight bar at 3 pressure levels (0, 20, 60 mm Hg). Calibration errors were classified into 3 categories: +/-0.5 mm Hg, +/-1.0 mm Hg, or +/-1.5 mm Hg or more. Ophthalmologists using tonometers with calibration errors > +/-1.0 mm Hg were notified, with the suggestion that the tonometers be returned to the manufacturer for recalibration.

A total of 25 tonometers were evaluated. At the 20 mm Hg level, 17 (68%) fell within the manufacturer's recommended calibration range of +/-0.5 mm Hg, and 2 (8%) had errors of +/-1.0 mm Hg, resulting in 19 tonometers (76%) with errors of +/-1.0 mm Hg. Six tonometers (24%) were identified as having calibration errors greater than +/-1.0 mm Hg.

Calibration error of +/-1.0 mm Hg or less is clinically acceptable. Seventy-six percent of the Goldmann applanation tonometers in the Eye Care Centre were in the acceptable calibration range. As the number of acceptably calibrated tonometers was high, despite the infrequency of recent calibration, we feel that monthly evaluation of Goldmann tonometers is suitable.