Schiotz Scleral Intraocular Pressure Readings Predict Goldmann Applanation Readings Better Than Rebound Tonometry

PURPOSE: To study the agreement between scleral intraocular pressure (IOP) measurements using the Schiotz, Icare, and Icare PRO tonometers versus Goldmann applanation tonometer (GAT) in eyes with nonscarred corneas. METHODS: This cross-sectional study included 83 eyes of 55 subjects. The order of IOP estimation was the corneal GAT, followed by ICare, ICare PRO, and Schiotz on the corneal and temporal sclera. The agreement between different tonometers and the 95% limits of agreement (LoA) were assessed using Bland-Altman plots. The repeated measures correlation coefficient was calculated between GAT IOP and scleral Schiotz IOP, and the 95% confidence intervals were calculated by the bootstrap method. The linear mixed effects model was used (adjusted for both eyes of the subjects) to generate an equation to predict GAT IOP from scleral Schiotz IOP. The prediction model was validated with new data from 60 eyes. Statistical analyses were performed using "R" software (version 3.3.2). RESULTS: Comparing the scleral IOP measurements, the mean IOP difference (95% LoA) was the lowest with Schiotz, underestimating by -1.21 mm Hg (7.32, -9.74). Both ICare and ICare PRO significantly overestimated GAT IOP: ICare, 24.6 mm Hg (53.2, -3.97); and ICare PRO, 21.56 mm Hg (52.9, -9.79). The correlation coefficient between scleral Schiotz and GAT IOP was 0.92 (95% bootstrap confidence interval: 0.89, 98). The derived predictive equation was (Equation is included in full-text article.). The mean difference between the predicted GAT IOP and the actual GAT IOP was 0.96 mm Hg with narrow LoA (+1.79, -3.71), validating the prediction model. CONCLUSIONS: Among the tonometers tested, the scleral IOP measurements with Schiotz had the best agreement with the GAT although LoA were wide. The predictive equation may have great potential to predict GAT IOP from scleral IOP readings in eyes with scarred/prosthetic corneas.