Demographics and Medical Variables
Demographic information including age at the most recent clinic visit, gender, ethnicity and type of insurance as proxy for socioeconomic status were extracted from the EHR. Baseline disease information included the genotype, baseline hemoglobin level, hematocrit and absolute reticulocyte count. Treatment variables included both hydroxyurea and chronic transfusion therapy. Hydroxyurea was included as a treatment if the medication had been prescribed to the patient although compliance of the medication was not factored. Chronic transfusion therapy was for patients with history of stroke, abnormal TCD, severe acute chest syndrome or severe pain. Disease complications such as acute chest syndrome, stroke and avascular necrosis were extracted by hand from the medical record.