Pooled analysis of the 10 studies documenting TCD measurement before and after hydroxyurea therapy in children with HbSS or HbSβ⁰thalassemia. This meta-analysis demonstrates the average drop in TCD measurement after starting hydroxyurea therapy of 21 cm/s (95% confidence interval [CI], 14.8-29.0). The plot also suggests that the decrease in TCD measurements can be seen as early as 3 months after the start of hydroxyurea therapy with a sustained impact of hydroxyurea therapy on decreasing TCD measurements for at least 36 months. The analysis is updated from a previous one by DeBaun and Kirkham,⁵⁸  plus additional references.^{51-57,59,60,165}  ♦ represents the pooled estimate from a random-effect model, the edges of the diamonds represent the 95% CI; ▪ represents individual studies. MTD, maximum tolerated dose.