PURPOSE: To investigate the variability in the measured calibration error with continued usage of Goldmann applanation tonometers (GATs) having unacceptable calibration error. METHODS: The study included 132 slit-lamp mounted Goldmann tonometers (Model AT 900 C/M; Haag-Streit, Switzerland). A single observer twice checked a randomly selected set of 25 instruments on 2 consecutive days to determine the intraobserver agreement in the measurement of GAT calibration error. The same observer prospectively checked all the instruments between 8 and 9 AM on any given day at all testing levels namely 0, 20, and 60 mm Hg and rechecked the faulty instruments (calibration error more than ± 2 mm Hg at any testing level) 2 times more on the same day between 12 noon and 1 PM and 4 and 5 PM. RESULTS: The single measures intraclass correlation coefficients for the intraobserver agreement at the 20 mm Hg testing level were 0.78 for positive error and 0.83 for negative error. Twenty-eight (21%) instruments were faulty at any testing level. Nineteen (14%) were faulty at the clinically most important 20 mm Hg testing level. The maximum observed variability in the positive and negative calibration error at any testing level was +4 and -23 mm Hg, respectively. Fifteen (53%) faulty instruments had high variability (≥ 2 mm Hg) in the calibration error at any testing level. CONCLUSIONS: The calibration error of faulty GATs can frequently have a high variability. One should avoid estimating the true intraocular pressure from a faulty GAT by instinct.
PURPOSE: To investigate the variability in the measured calibration error with continued usage of Goldmann applanation tonometers (GATs) having unacceptable calibration error. METHODS: The study included 132 slit-lamp mounted Goldmann tonometers (Model AT 900 C/M; Haag-Streit, Switzerland). A single observer twice checked a randomly selected set of 25 instruments on 2 consecutive days to determine the intraobserver agreement in the measurement of GAT calibration error. The same observer prospectively checked all the instruments between 8 and 9 AM on any given day at all testing levels namely 0, 20, and 60 mm Hg and rechecked the faulty instruments (calibration error more than ± 2 mm Hg at any testing level) 2 times more on the same day between 12 noon and 1 PM and 4 and 5 PM. RESULTS: The single measures intraclass correlation coefficients for the intraobserver agreement at the 20 mm Hg testing level were 0.78 for positive error and 0.83 for negative error. Twenty-eight (21%) instruments were faulty at any testing level. Nineteen (14%) were faulty at the clinically most important 20 mm Hg testing level. The maximum observed variability in the positive and negative calibration error at any testing level was +4 and -23 mm Hg, respectively. Fifteen (53%) faulty instruments had high variability (≥ 2 mm Hg) in the calibration error at any testing level. CONCLUSIONS: The calibration error of faulty GATs can frequently have a high variability. One should avoid estimating the true intraocular pressure from a faulty GAT by instinct.