Check your tonometer for calibration error at least once a month, says a new study in Junes Journal of Glaucoma.1
Goldmann applanation tonometersthe gold standard for measuring intraocular pressure (IOP)can go out of whack more quickly than the manufacturer (Haag-Streit, in this study) suggests, according to this study.
Researchers at the Royal Victoria Infirmary, in Newcastle-upon-Tyne, England, performed calibration error checks on all tonometers in the department of ophthalmology. At the end of the four-month study, fewer than 10% of the tonometers fell within the manufacturers recommended calibration error range (-0.5mm Hg to 0.5mm Hg).
The tolerance set by the manufacturer is too strict for use in the clinical environment, the study says. Rather, the researchers say, calibration error of less than 2.5mm Hg is clinically acceptable.
Using this number, the researchers found that about one-third to one-half of their tonometers met this criterion. Tonometers with calibration error greater than 2.5mm Hg should be returned to the manufacturer for re-calibration, they say.
The researchers found that tonometers are more likely to deviate into the positive range than the negative, which may lead to overestimating IOP measurements. Even so, accurate IOP measurement is crucial to glaucoma management, the study says. For example, the Early Manifest Glaucoma Trial showed that a 1mm Hg reduction in IOP could reduce the risk of nerve damage progression by 10%.2
Ideally, doctors should check for calibration error before each session, say the Royal Victoria researchers.
1. Sandhu SS, Chattopadhyay S, Birch MK, Ray-Chaudhuri N. Frequency of Goldmann applanation tonometer calibration error checks. J Glaucoma 2005 Jun;14(3):215-8.
2. Leske MC, Heijl A, Hussein M, et al.; Early Manifest Glaucoma Trial Group. Factors for glaucoma progression and the effect of treatment: the early manifest glaucoma trial. Arch Ophthalmol 2003 Jan;121(1):48-56.
