**4.2 Repeatability and reproducibility challenges in optical coherence tomography angiography**

OCTA's use is slowly becoming popular among clinicians for the diagnosis and monitoring of glaucoma. There are a number of commercially available OCTA devices

#### *The Role of Optical Coherence Tomography Angiography in Glaucoma DOI: http://dx.doi.org/10.5772/intechopen.110272*

including RTVue XR Avanti (Visionix USA Inc., previously Optovue Inc., Lombard, IL), Cirrus 5000 (Carl Zeiss Meditec Inc., Dublin, CA), PLEX Elite 9000 (Carl Zeiss Meditec Inc., Dublin, CA), and SPECTRALIS 2 (Heidelberg Engineering, Heidelberg, Germany). These devices all vary in terms of scan speed, wavelengths, eye tracking, image size, and vessel delineation algorithms. Differences in scan parameters and methodologies have led to discrepancies between images that limit direct comparisons across multiple centers and studies.

Several investigators have evaluated intra and inter-visit repeatability and of OCTA measurements in the ONH, peripapillary, and macular regions. Manalastas et al. compared the reproducibility of VD measurements between healthy and glaucoma eyes using the coefficient of variation (CV) the relative dispersion of data points around the mean, with a lower number indicating less variation [53]. The authors found that the CV of intra- and inter-visit global VD measures in healthy eyes ranged from 2.5–9.0% in macular scans and 1.8–3.2% in ONH scans, while the CV was higher in glaucoma (3.2–7.9% in macular scans vs. 2.3–4.1% in ONH scans. Venugopal et al. described similar results for peripapillary region (normal 2.5–4.4% vs. glaucoma 2.6–6.6%) and macular region (normal 3.3–4.7% vs. glaucoma 3.7–5.6%) [51]. Together, these studies suggest that glaucoma patients have sparser VD with slightly poorer reproducibility than healthy subjects [52].

Manalastas et al. examined the intra-visit and inter-visit reproducibility of SD-OCT RNFL thickness and GCC thickness in the scans from the prior studies [53]. The CVs of the global RNFL and GCC thickness were ≤ 4%, and the superior and inferior RNFL and GCC ≤ 3.5% in both healthy and glaucoma eyes. In agreement with the prior VD studies, glaucomatous eyes had worse RNFL and GCC reproducibility than healthy eyes (p < 0.001). The reproducibility of the global VD measures provided by OCTA is slightly worse than that of RNFL and GCC measures traditionally found in OCT.

AlSalem et al. studied reproducibility between patients across all stages of POAG and concluded that the range of CV for structural properties was greater than VD as glaucoma progressed [9]. Thus, as VD provides better reproducibility than structural values it may be a more consistent metric in severe POAG.
