Improving Scientific Rigor of Renal Clinical Endpoints for Sickle Cell Anemia

200 pediatric and 200 adult SCA participants will undergo mGFR using iohexol at baseline and at one year. On the same day, participants will undergo clinical and laboratory variables important for developing an eGFR equation. From baseline data, the investigators will perform a training and validation cohort of biologically relevant predictor variables to develop a novel equation. Then the investigators will compare the novel SCA eGFR equation to the established eGFR equation. From one year data, the investigators will determine the longitudinal concordance between pediatric and adult novel and standard eGFR equations with mGFR and variables that may alter longitudinal inferences. In a subset of 30 adult participants, the investigators will also evaluate the correlation between mGFR using iohexol and a novel measure of mGFR that is more time efficient