OBJECTIVES: With normal clinical use, Goldmann applanation tonometers frequently develop calibration errors. Only the manufacturer can perform recalibration. This study aimed to assess whether intraocular pressure (IOP) measured by Goldmann applanation tonometers with known small calibration errors could be adjusted to reflect true IOP to allow continued clinical use. DESIGN: Evaluation of diagnostic test. PARTICIPANTS: Patients under regular review who had undergone previous applanation tonometry. METHODS: Patients with a range of IOPs underwent IOP measurement using a gold standard 0-error tonometer and tonometers with known calibration errors in a randomized blind fashion. The calibration errors of the tonometers ranged 0 to +5 mmHg. MAIN OUTCOME MEASURES: Intraocular pressure. RESULTS: For the first part of the study, 125 eyes of 125 patients with a mean IOP of 18.5 mmHg (range, 8-43 mmHg) were tested. Mean IOP measured by the tonometer with an error of +1 mmHg was +1.0 (95% confidence interval [CI], 0.3-1.7 mmHg; P = 0.0076, compared with gold standard 0-error), with the +2 mmHg error was +1.2 (95% CI, 0.8-1.7 mmHg; P<0.0001), with the +3 mmHg error was +1.6 (95% CI, 1.2-1.9 mmHg; P<0.0001), with the +4 mmHg error was +3.6 (95% CI, 2.9-4.2 mmHg; P<0.0001), and with the +5 mmHg error was +3.3 (95% CI, 2.9-3.8 mmHg; P<0.0001). In the second part of the study, IOP measured by each of the tonometers with +2 mmHg error was +0.6 mmHg (95% CI, 0.1-1.1 mmHg; P = 0.0241), +1.5 mmHg (95% CI, 1.0-2.0 mmHg; P<0.0001), and +1.5 mmHg (95% CI, 1.9-2.1 mmHg; P<0.0001). CONCLUSIONS: There is a relationship between calibration error and clinical error in IOP measured, but it is not a one-to-one relationship. The error overestimates IOP and is consistent over a clinical range of IOPs. In certain circumstances where resources are limited, it may be clinically acceptable to use tonometers with calibration errors of less than +3 mmHg, because they do not overestimate IOP by more than 2 mmHg. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.
OBJECTIVES: With normal clinical use, Goldmann applanation tonometers frequently develop calibration errors. Only the manufacturer can perform recalibration. This study aimed to assess whether intraocular pressure (IOP) measured by Goldmann applanation tonometers with known small calibration errors could be adjusted to reflect true IOP to allow continued clinical use. DESIGN: Evaluation of diagnostic test. PARTICIPANTS: Patients under regular review who had undergone previous applanation tonometry. METHODS:Patients with a range of IOPs underwent IOP measurement using a gold standard 0-error tonometer and tonometers with known calibration errors in a randomized blind fashion. The calibration errors of the tonometers ranged 0 to +5 mmHg. MAIN OUTCOME MEASURES: Intraocular pressure. RESULTS: For the first part of the study, 125 eyes of 125 patients with a mean IOP of 18.5 mmHg (range, 8-43 mmHg) were tested. Mean IOP measured by the tonometer with an error of +1 mmHg was +1.0 (95% confidence interval [CI], 0.3-1.7 mmHg; P = 0.0076, compared with gold standard 0-error), with the +2 mmHg error was +1.2 (95% CI, 0.8-1.7 mmHg; P<0.0001), with the +3 mmHg error was +1.6 (95% CI, 1.2-1.9 mmHg; P<0.0001), with the +4 mmHg error was +3.6 (95% CI, 2.9-4.2 mmHg; P<0.0001), and with the +5 mmHg error was +3.3 (95% CI, 2.9-3.8 mmHg; P<0.0001). In the second part of the study, IOP measured by each of the tonometers with +2 mmHg error was +0.6 mmHg (95% CI, 0.1-1.1 mmHg; P = 0.0241), +1.5 mmHg (95% CI, 1.0-2.0 mmHg; P<0.0001), and +1.5 mmHg (95% CI, 1.9-2.1 mmHg; P<0.0001). CONCLUSIONS: There is a relationship between calibration error and clinical error in IOP measured, but it is not a one-to-one relationship. The error overestimates IOP and is consistent over a clinical range of IOPs. In certain circumstances where resources are limited, it may be clinically acceptable to use tonometers with calibration errors of less than +3 mmHg, because they do not overestimate IOP by more than 2 mmHg. FINANCIAL DISCLOSURE(S): The author(s) have no proprietary or commercial interest in any materials discussed in this article.