Measuring accurate IOPs: Does correction factor help or hurt?

Abstract

Purpose: To evaluate if using the Ehlers correction factor on the intraocular pressure (IOP) measured using the Goldmann applanation tonometer (GAT) improves its agreement with the PASCAL dynamic contour tonometer (DCT).

Patients and methods: A total of 120 eyes of 120 individuals were examined. Participants underwent IOP measurement with both the DCT and the GAT and central corneal thickness measurement. The Ehlers correction factor was applied on the GAT IOP measurements to calculate Ehlers-corrected GAT IOP. The agreement between the DCT and GAT, and DCT and Ehlers-corrected GAT IOP was analyzed. The analyses were repeated by stratifying the data by race.

Results: The mean IOP of the GAT, DCT, and the Ehlers-corrected GAT was 15.30, 16.78, and 14.68 mmHg, respectively. The agreement as assessed by Bland-Altman plot for the GAT with the DCT and DCT and Ehlers-corrected GAT IOP was +4.1 to -6.9 and +4.15 to -8.25 mmHg, respectively. The results were similar even when stratifying the data by race.

Conclusion: Using Ehlers correction factor to account for the effect of corneal parameters on the IOP measured by the GAT worsens the agreement with the DCT. This effect remains even when stratifying the data by race.

Keywords: Goldmann applanation tonometer; central corneal thickness; dynamic contour tonometer; intraocular pressure; tonometric correction factors.

Figure 1

Scatterplot showing the association between the central corneal thickness and intraocular pressure measured by the Goldmann applanation tonometer. There is a linear positive trend with measured intraocular pressure being lesser in eyes with lower-than-average central corneal thickness and greater with higher-than-average central corneal thickness.

Figure 2

Scatterplot showing the association between the central corneal thickness and intraocular pressure measured by the PASCAL dynamic contour tonometer. The trend line is flat indicating no association between the two variables.

Figure 3

Scatterplot showing the association between the central corneal thickness and the difference in intraocular pressure measured by the Goldmann applanation tonometer and the PASCAL dynamic contour tonometer. There is a linear positive trend with difference in intraocular pressure being negative in eyes with lower-than-average central corneal thickness and positive with higher-than-average central corneal thickness. This indicates that the difference in intraocular pressure may be accounted in part by central corneal thickness.

Figure 4

A Bland–Altman plot assesses the agreement between intraocular pressure values measured using the Goldmann applanation tonometer and the PASCAL dynamic contour tonometer. The upper limit of agreement (ULA) and the lower limit of agreement (LLA) are wide, indicating that agreement between the two tonometers is poor. The bias line is negative, indicating that the PASCAL dynamic contour tonometer on average measures greater intraocular pressure when compared with the Goldmann applanation tonometer.

Figure 5

A Bland–Altman plot assesses the agreement between intraocular pressure values measured using the PASCAL dynamic contour tonometer and the Ehlers intraocular pressure calculated from the Goldmann intraocular pressure measurements. The upper limit of agreement (ULA) and the lower limit of agreement (LLA) are wide, indicating that agreement between the intraocular pressure values is poor. The bias line is negative, indicating that the Ehlers intraocular pressure on average is lower than the intraocular pressure measured by the PASCAL dynamic contour tonometer.

Figure 6

Scatterplot showing the residual association between the Central corneal thickness and difference between the Ehlers intraocular pressure and the PASCAL dynamic contour tonometer; a linear negative trend is visible.