Comparison of Kang's and Littmann's Methods of Correction for Ocular Magnification in Circumpapillary Retinal Nerve Fiber Layer Measurement

PURPOSE: To assess and compare the accuracy of Kang's method for ocular magnification correction in circumpapillary retinal nerve fiber layer (cpRNFL) thickness measurement with that of the currently used Littmann's method. METHODS: A total of 148 eyes of 148 healthy participants underwent cpRNFL measurement without correction and with correction by Littmann's method using spectral-domain optical coherence tomography. Correction of ocular magnification by Kang's method is based on the observed uncorrected cpRNFL thickness. The accuracy of Kang's method was compared with that of Littmann's method for magnification correction by using the Pearson product-moment correlation coefficient (r) and Bland-Altman analysis. RESULTS: A total of 132 eyes were assessed. The two methods used to correct the global cpRNFL thickness were strongly correlated (r = 0.940, P < 0.01), and the mean difference between the two methods was -0.4 μm, with 95% limits of agreement (LoA) of ±6 μm without systemic bias (P > 0.05). Although the cpRNFL thicknesses corrected with the two methods at quadrant and 12-o'clock sectors showed a strong correlation (r > 0.731, P < 0.01), high LoAs ranging from ±13.3 to ±27.9 and systemic biases were observed at nasal and inferior sectors. CONCLUSION: No difference was found between Kang's and Littmann's methods for correction of ocular magnification in global cpRNFL thickness measurement. However, with regard to magnification correction for sectoral cpRNFL thickness, further scrutiny of Kang's method is warranted, even in healthy participants, because of the high variability between Littmann's and Kang's methods.