MIRCA readouts capture low red cell deformability in individuals on chronic transfusion therapy, which can be improved with the in vitro addition of osivelotor or voxelotor. (A-D) A nonlinear relationship of HbS% with MIRCA (A-B) and LoRRca (C-D) readouts. NOI and HOI values worsen, EI max values remain stable, and EI Min values improve as HbS values decrease below 40% after blood transfusion. The statistically significant association of HOI and HbS (P = .001) could be due to lower HbF values in this group compared with the patients with HbS between 40 and 80 (median HbF = 0% [range, 0-9.3] vs 12.2% [range, 6.5-20], P = .013). (E-F) No significant difference in the median NOI (E) and HOI (F) of individuals with HbS <40% vs those with HbS >90% and not on HU MTD (P = .3 and P = .3, respectively). To test the impact of in vitro addition of antisickling agents on deformability, RBCs from 12 individuals with HbSS were treated with 1.66 mM osivelotor (n = 8) or voxelotor (n = 6) (G) dissolved in DMSO or with DMSO alone, incubated at 25°C for 1 hour and resuspended in 1.5% MBS in 1× PBS and run on the MIRCA. Osivelotor- and voxelotor-treated samples were found to have 17.3% and 10.8% lower HOI values compared with DMSO-treated controls (P = .008 and .03, respectively, Wilcoxon signed rank test). However, osivelotor and voxelotor sample HOI values were still elevated by 3.7% and 7.0% compared with 4 healthy controls (P = .008 and .006, respectively, Mann-Whitney U test). ∗P ≤ .05; ∗∗P ≤ .01. ns, not significant.