Correction factors for Goldmann Tonometry

The purpose of the study was to evaluate the in vitro accuracy of correction factors in decreasing the error in the intraocular pressure (IOP) measurements obtained using the Goldmann Applanation Tonometer (GAT).

Nineteen donor corneas, from individuals aged between 57 and 99 years (mean 75.7 years, standard deviation±11.4 years) were subjected to posterior pressure simulating in vivo true IOP (IOPT) using an inflation test rig. Central corneal thickness and corneal curvature were measured. The posterior pressure was set at 25 different pressure levels between 5 and 45 mm Hg and IOP was measured using the GAT. Five different correction equations were applied to the IOP measurements obtained using the GAT to determine corrected IOP. The multiparameter correction equations applied were derived by Elsheikh, Ehlers, Chihara, Shimmyo et al, and Orssengo and Pye. The differences between IOPT and the IOP measured using the GAT were recorded as uncorrected errors, whereas the differences between IOPT and each of the corrected IOP were the tonometry errors after correction.

The mean and standard deviation of error in tonometry before correction was +2.25±0.62 mm Hg. The mean errors in tonometry after correction using the Elsheikh and Chihara equations were +0.78±0.62 and +1.08±0.61 mm Hg, respectively. The mean errors in tonometry for the Ehlers, Shimmyo et al, and Orssengo and Pye equations were negative, indicating an overcorrection; the values were -0.75±2.28, -1.27±1.85, and -0.77±1.83 mm Hg, respectively.

The Elsheikh and the Chihara et al's equations considerably decreased error in IOP measurements obtained by the GAT when compared with IOPT and were more consistent than other correction equations. The 2 equations may be of clinical utility in obtaining estimates of IOPT.