Agreement between total corneal astigmatism calculated by vector summation and total corneal astigmatism measured by ray tracing using Galilei double Scheimpflug analyzer.

PURPOSE To evaluate the agreement between total corneal astigmatism calculated by vector summation of anterior and posterior corneal astigmatism (TCAVec) and total corneal astigmatism measured by ray tracing (TCARay). METHODS This study enrolled a total of 204 right eyes of 204 normal subjects. The eyes were measured using a Galilei double Scheimpflug analyzer. The measured parameters included simulated keratometric astigmatism using the keratometric index, anterior corneal astigmatism using the corneal refractive index, posterior corneal astigmatism, and TCARay. TCAVec was derived by vector summation of the astigmatism on the anterior and posterior corneal surfaces. The magnitudes and axes of TCAVec and TCARay were compared. The Pearson correlation coefficient and Bland-Altman plots were used to assess the relationship and agreement between TCAVec and TCARay, respectively. RESULTS The mean TCAVec and TCARay magnitudes were 0.76±0.57D and 1.00±0.78D, respectively (P<0.001). The mean axis orientations were 85.12±30.26° and 89.67±36.76°, respectively (P=0.02). Strong correlations were found between the TCAVec and TCARay magnitudes (r=0.96, P<0.001). Moderate associations were observed between the TCAVec and TCARay axes (r=0.75, P<0.001). Bland-Altman plots produced the 95% limits of agreement for the TCAVec and TCARay magnitudes from -0.33 to 0.82D. The 95% limits of agreement between the TCAVec and TCARay axes was -43.0 to 52.1°. CONCLUSION The magnitudes and axes of astigmatisms measured by the vector summation and ray tracing methods cannot be used interchangeably. There was a systematic error between the TCAVec and TCARay magnitudes.