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Background  To investigate the repeatability of superficial retinal vessel density measurements in healthy eyes with long axial length (AL) using optical coherence tomography angiography (OCTA).    Methods  There were 60 eyes of 31 volunteers enrolled in this cross-sectional observational study. All subjects underwent OCTA, AL and refraction test. The enrolled eyes were divided into the long AL group (26 mm ≤ AL < 28 mm) and normal AL group (22 mm ≤ AL < 26 mm). The vessel length density (VLD), perfusion density (PD), and fovea avascular zone (FAZ) of the superficial retinal vessel were evaluated. Repeatability was assessed by intraclass correlation coefficients (ICCs) and Bland-Altman analysis. Pearson’s r correlation was used to analyze the relation of AL and the absolute difference between two measurements.    Results  The 3 × 3 mm scan pattern showed good repeatability with all ICCs over 0.7. For all parameters of all scan patterns, the ICCs of the normal AL group were distinctly higher than those of the long AL group; this finding was also confirmed by Bland-Altman analysis. The correlation analysis of AL and repeatability of OCTA parameters showed significant negative correlations between the ALs and repeatability of VLD in 6 × 6 mm inner ring ( r  2  = 0.13,  p  = 0.01), VLD in 6 × 6 mm outer ring ( r  2  = 0.09,  p  = 0.02) and PD in 6 × 6 mm outer ring ( r  2  = 0.08,  p  = 0.03).    Conclusions  The AL and the scanned area will both affect the repeatability of superficial retinal vessel density measurements in OCTA.    Electronic supplementary material  The online version of this article (10.1186/s12886-018-0992-y) contains supplementary material, which is available to authorized users.

The repeatability of superficial retinal vessel density measurements in eyes with long axial length using optical coherence tomography angiography