Evaluation of correction formulas for tonometry. The Goldmann applanation tonometry in approximation to dynamic contour tonometry

Accurate determination of intraocular pressure (IOP) is essential for correct management of glaucoma. Goldmann applanation tonometry (GAT) is the gold standard for measuring IOP despite its limitations due to its dependence on corneal properties. With the aim of improving the accuracy of GAT readings, several correction formulas have been developed. The aim of this study was to investigate the accuracy and clinical relevance of five correction equations for GAT. Prospective study of 112 glaucoma patients at the University Hospital and Talacker Eye Center, Zurich, Switzerland. The IOP was measured with GAT and dynamic contour tonometry (DCT) in randomized order. The GAT readings were adjusted with five correction equations. The primary study endpoint was the degree of concordance between corrected GAT and DCT readings. A discordance of <= 2 mm Hg was defined as significant. The association between discordant IOP measurements and central corneal thickness (CCT) was the secondary study endpoint. The mean patient age was 66 +/- 13 years (60% females and 56% left eyes). The mean IOP was 17.0 mm Hg for GAT and 20.3 mm Hg for DCT, with a discordance of 3.3 mm Hg between GAT und DCT. The discordances between DCT and the corrected values ranged from 2.7 to 5.4 mm Hg. Spearman's rank testing showed a positive correlation between CCT and the discordances of all correction equations and a negative correlation between CCT and the discordance of DCT and GAT. The use of GAT correction formulas involves the risk of creating significant error. The correction equations examined showed extensive scatter and resulted in mean IOP values that were lower than the IOP initially measured by GAT. Thus the use of any correction equation may delay diagnosis of glaucoma and should be avoided.