Reproducibility of Optovue RTVue Optical Coherence Tomography Retinal Thickness Measurements and Conversion to Equivalent Zeiss Stratus Metrics in Diabetic Macular Edema

Abstract

Purpose: To evaluate the reproducibility of central subfield thickness (CST) and volume measurements from optical coherence tomography (OCT) images obtained with Zeiss Stratus and Optovue RTVue, and formulate equations to convert these measurements from RTVue to 'equivalent' Stratus values.

Methods: Cross-sectional observational study from 309 eyes of 167 participants with diabetes and at least one eye with central-involved diabetic macular edema (DME; Stratus CST ≥ 250 μm) that underwent two replicate Stratus scans followed by two replicate RTVue scans centered on the fovea.

Results: The Bland-Altman coefficient of repeatability for relative change in CST (the degree of change that could be expected from measurement variability) was not significantly different on Stratus and RTVue scans (10% and 16%, respectively). The replicate Stratus CST was within 10% of the initial Stratus measurement 93% of the time; the CST conversion equation predicted a Stratus value calculated from the observed RTVue value within 10% of the observed Stratus thickness 91% of the time. Bland-Altman limit of agreement for relative change in CST between measurements observed on different machines was 23%, comparing predicted versus actual Stratus measurement.

Conclusions: RTVue thickness reproducibility appears similar to Stratus. Conversion equations to transform RTVue measurements to Stratus-equivalent values within 10% of the observed Stratus RT are feasible. CST changes greater than 10% when using the same machine or 20% when switching from Stratus to RTVue, after conversion to Stratus equivalents, are likely due to a true change beyond measurement error.

Translational relevance: Conversion equations to translate central retinal thickness measurements between OCT instruments is critical to clinical trials.

Keywords: diabetic macular edema; optical coherence tomography; retinal thickness measurements.

Figure 1.

Bland-Altman plots of the differences versus the means of the automated test–retest values for the given parameter, within each machine. Solid reference line indicates mean difference; dashed lines indicate the coefficient of repeatability. (A) Stratus CST results. (B) RTVue CST results. Two influential data points are indicated with an asterisks and are not included in the calculation of the coefficients of repeatability. (C) Stratus retinal volume results. (D) RTVue retinal volume results.

Figure 2.

Bland-Altman plots of the differences between values on machines (RTVue minus Stratus) versus the means of the automated Stratus test–retest values, for each measurement. Solid reference line indicates mean difference; dashed lines indicate the limits of agreement. Eyes meeting one of several outlier criteria are indicated with an asterisks and are not included in the calculations of the limits of agreement. (A) Central subfield thickness results, (B) retinal volume results.

Figure 3.

Bland-Altman plots of the relative (%) differences between the predicted Stratus value (based on the RTVue value and using the conversion equation) minus the corresponding observed Stratus value, for each measurement. Solid reference line indicates mean difference; dashed lines indicate the limits of agreement. (A) Central subfield thickness results, (B) retinal volume results.