(A) No significant difference was observed in OI when comparing the on-HU subjects (N = 12) to off-HU subjects (N = 6) among the study HbSS population. (B) HU responders (N = 5) had significantly lower OIs compared to HU non-responders (N = 7, P = 0.023, Mann-Whitney). HU responders (N = 5) among the on-HU HbSS subjects had relatively lower OIs compared to off-HU HbSS subjects (N = 6, P = 0.083, Mann-Whitney). No significant difference was observed in the OI between HU non-responders (N = 7) and off-HU HbSS subjects (N = 6). Error bars represent standard deviation (SD). ns, no significance.