Mean TRV at baseline and 2-year follow-up evaluation. Change in mean TRV in the entire cohort and within each subgroup at baseline and at the 2-year follow-up evaluation. Group 1: not on DMT at baseline or at 2-year evaluation; group 2: not on DMT at baseline evaluation but started on hydroxyurea prior to the 2-year evaluation; group 3: on hydroxyurea at baseline and 2-year evaluation; group 4: on monthly blood transfusion at baseline and 2-year evaluation. One outlier in group 1 at baseline with TRV of 0.05 m/s is not shown. *P ≤ .05, **P ≤ .01. NS, not significant.