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Proliferative sickle cell retinopathy (PSCR) is the most important ocular manifestation of sickle cell disease (SCD), but understanding of its pathophysiology remains incomplete. Red blood cell (RBC) deformability has been identified as a critical factor in SCD and is intrinsically related with cell sphericity index (SI). The aim of this study was to develop an algorithm that automatically analyzes defocusing microscopy (DM) images, providing biomechanical parameters of individual RBCs, including cellular SI. Furthermore, association of RBC SI with severity of PSCR in individuals with HbSC-SCD was also investigated. We determined the SI of 255 RBCs from 17 individuals using DM. Nine individuals (53%) were healthy and eight (47%) had HbSC-SCD. Mean SI was significantly lower in SCD than in control group (0.69 versus 0.73), and also in severe than in mild PSCR (0.67 versus 0.70). Lower SI was also associated with more severe retinopathy (ROC AUC = 0.85) and the proportion of participants with SI <= 0.69 was significantly greater in severe than in mild PSCR (100% versus 25%). In conclusion, lower RBC SI determined by DM was associated not only with the presence of SCD, but also with more severe PSCR, possibly being a surrogate biomarker of disease severity., Comment: 21 pages, 5 figures, 2 tables, Based on PhD thesis