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Objective Hydroxyurea lowers the incidence of vaso‐occlusive pain crises (VOC) and acute chest syndrome (ACS) among children with sickle cell anemia (SCA). Our objective was to assess the relationship between levels of adherence to hydroxyurea and clinical outcomes among children and adolescents with SCA. Methods This retrospective cohort study included Medicaid data (2005–2012) from Florida, Illinois, Louisiana, Michigan, South Carolina, and Texas. The study population consisted of children 1–17 years old with SCA enrolled in Medicaid for 3 years. Among children that initiated hydroxyurea, the medication possession ratio (MPR) was calculated as the proportion of days covered by hydroxyurea. Six months after initiation of hydroxyurea, clinical outcomes were assessed through the end of the study period: numbers of VOC‐related inpatient admissions and emergency department visits, and encounters for ACS. Multivariable Poisson models were used to predict outcomes by MPR quartile adjusting for previous healthcare utilization, state, and age. Results Hydroxyurea was initiated by 515 children. The median MPR was 0.53 (interquartile range = 0.3–0.8). The annual median number of visits was 0.0 for ACS, 1.3 for VOC‐related emergency department, and 1.4 for VOC‐related inpatient admissions. For each outcome, the highest quartile of MPR had the lowest predicted count; this difference was significant for ACS visits when compared with the lowest quartile of MPR. Conclusion This study demonstrated a high level of adherence (>75%) was essential to achieve a lower incidence of common negative clinical outcomes. Further, moderate and severe hydroxyurea nonadherence may be more common than previously appreciated among children, emphasizing the importance of developing and testing innovative strategies to increase adherence.

ImportanceYouths with sickle cell anemia (SCA) are at risk of pain crises, stroke, and early death. Complications can be reduced by the oral disease-modifying medication hydroxyurea, and in 2014, the National Heart, Lung, and Blood Institute published revised guidelines that hydroxyurea should be offered to youths aged 9 months and older with SCA regardless of disease severity.ObjectiveTo describe changes in hydroxyurea use among youths with SCA before and after release of the National Heart, Lung, and Blood Institute guidelines.Design, Setting, and ParticipantsThis cross-sectional study was conducted using administrative data from 2010 to 2018 from Michigan and New York State (NYS) Medicaid programs. The study population included youths aged 1 to 17 years with SCA enrolled in the Michigan or NYS Medicaid programs for at least 1 year (Michigan: 2010-2018; NYS: 2012-2018). Youths with SCA were identified using validated claims–based definitions. Data were analyzed from June to October 2020.Main Outcomes and MeasuresThe main outcome was hydroxyurea use characterized as mean annual counts of days’ supply of filled hydroxyurea prescriptions. Rates of hydroxyurea use over time were assessed using regression models (Michigan: zero-inflated negative binomial; NYS: negative binomial). Models included indicators for periods before and after guideline release.ResultsA total of 4302 youths with SCA (2236 males [52.0%]; 2676 born 2005-2017 [62.2%]; 150 Hispanic [3.5%], 2929 non-Hispanic Black [68.0%], and 389 non-Hispanic White [9.0%]) contributed 12 565 person-years. The mean (SD) annual days’ supply of hydroxyurea was 47.2 (93.6) days per youth in Michigan and 97.4 (137.0) days per youth in NYS. In Michigan, there was an increase in the odds of having nonzero days’ supply after the guidelines were released (odds ratio, 1.52; 95% CI, 1.07-2.14). In NYS, no change was seen in the mean days’ supply of filled hydroxyurea.Conclusions and RelevanceThese findings suggest that hydroxyurea was substantially underused among youths with SCA, despite establishment as the primary disease-modifying therapy for SCA, and that there was incomplete clinician or patient uptake of newly released guidelines. Results suggest that expanding use of hydroxyurea may require a multifaceted approach that includes addressing multiple system- and patient-level barriers.