Calculating the predicted retinal thickness from spectral domain and time domain optical coherence tomography - comparison of different methods.

Purpose: To compare the accuracy of different methods of calculating predicted central retinal thickness values in order to allow comparison between results of spectral-domain optical coherence tomography (SD-OCT) and time-domain OCT (TD-OCT) devices.

Methods: In a prospective cohort study, 100 consecutive healthy individuals without ocular disease underwent sequential scanning with SD-OCT (Spectralis OCT) and TD-OCT (Stratus OCT). A group of 60 eyes was used to generate the conversion equations, which were tested on the remaining 140 eyes. Four equations were used: 1. Mean difference between SD-OCT and TD-OCT; 2. Multiplying a ratio by the original retinal thickness; 3. Linear regression analysis using retinal thickness; and 4. Regression analysis using retinal thickness and spherical equivalent. All four methods were used to calculate predicted SD-OCT values from TD-OCT measurements, and vice versa.

Results: For all four equations, the predicted SD-OCT central retinal thickness values were similar to the actual SD-OCT, with mean difference ranging from 0.78 to 1.01 μm, and intraclass correlation coefficients >0.88. Both regression equations and mean difference showed greater accuracy, with variation between calculated and actual retinal thickness values ≤5 μm in 60% of eyes. In contrast, the ratio method was less accurate, with 15.8 % of eyes showing differences >15 μm. Similar results were found for predicted TD-OCT values.

Conclusions: Several methods can be used to convert central retinal thickness values from SD-OCT to the predicted TD-OCT value, or vice versa, with high degrees of accuracy and reliability. These methods may allow comparison of OCT values from SD-OCT and TD-OCT devices in clinical trials and standard patient care.