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Editors Selection IGR 9-1

Tonometry: Tonometer calibration

Comment by James Brandt on:

22659 Measurement of Goldmann applanation tonometer calibration error, Choudhari NS; George R; Baskaran M et al., Ophthalmology, 2009; 116: 3-8

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Over the last decade, ophthalmologists have come to recognize that our techniques to measure intraocular pressure (IOP) are far less precise and accurate than previously thought. In large part this has been due to renewed interest in central corneal thickness (CCT) brought on by clinical trials that demonstrated significant artifact in Goldmann Applanation Tonometry (GAT). The GAT is no longer on a pedestal as a 'gold standard' instrument. Combined with the recognition that the GAT is less precise and

Clinicians should have a protocol in the office to regularly verify calibration of their Goldmann tonometers

accurate than previously believed has been an increase in the sophistication of ophthalmologists in their understanding of statistics and measurement error. We now recognize that IOP is a noisy value (it fluctuates minute to minute, hour to hour, asleep and awake, with body position, breath-holding and a long list of other physiologic parameters) and that a single snapshot of this noisy value provides limited clinical information. Couple this with the recognition that the instruments we use to measure IOP are in themselves noisy (with artifact introduced by CCT, corneal material properties, refractive surgery, etc.) the clinician is left wondering what to do.Until we have 24 hour monitoring of IOP with a technique less affected by corneal properties, all we can do is try to get the best performance out of the instruments we have. Surveys performed in the United Kingdom have shown that few clinicians have a protocol in place to verify calibration of the GATs in their offices. They should. Choudhari et al. (124) recently monitored the calibration error of 132 Haag-Streit GATs at the Sankara Nethralaya Eye Hospital in Chennai, India. They found that only 4% of GATs were within the manufacturer's recommended calibration error tolerance of ± 0.5 mmHg. Four months after the testing (and repair of the faulty instruments), 20% of the GATs were found to have calibration errors of more than ± 2 mHg. What is the take-home message from this study? Instead of leaving the tonometer calibration bar collecting dust in a drawer somewhere, clinicians should have a protocol in the office to regularly verify calibration of their Goldmann tonometers.

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