Optimizing Hydroxyurea Dosage With Pharmakokinetic in Patients Suffering of Moderate to Severe Sickle Cell Anemia

Inclusion Criteria (Group A and B) : * Have had confirmed diagnosis of SCD at CHU Sainte-Justine biochemistry lab with hemoglobin electrophoresis. * Be patients with SS, SBThal0. * Agree to take hydroxyurea for a period of 12 months * Be between age of 6months old and 18 years old. * Have consented for participation in the study. Inclusion Criteria (Group C) : * Have had confirmed diagnosis of SCD at CHU Sainte-Justine biochemistry lab with hemoglobin electrophoresis. * Be patients with SS, SBThal0. * Have taken hydroxyurea for a period of at least 12 months, and have received HU at a stable dose and at MTD for at least 6 months. * Be between age of 6months old and 18 years old. * Have consented for participation in the study. Exclusion Criteria: * Patients with sickle cell genotype other than SS or SBThal0 (SC, SBThal+, SE or SD) * Patients on chronic transfusion program * Patients have received a blood transfusion in the last 4 weeks of study enrollment. * Have received a hematopoietic stem-cell transplantation * Creatinine \>2x normal for age * ALT\>2x normal for age * Sexually active females unwilling to comply with reliable method of birth control * Pregnancy * Conditions which in the opinion of the investigator, would compromise participation in the study will be excluded.