OBJECTIVES : To determine if adherence to hydroxyurea is associated with vaso-occlusive crisis (VOC) events among sickle cell disease (SCD) patients.

METHODS : This was a retrospective study using Texas Medicaid claims data from 9/1/11-8/31/15. The index date was the date of hydroxyurea initiation. Patients who were between 2 and 62 years old, had ≥ 1 hydroxyurea prescription from 3/1/12-8/31/14, had no hydroxyurea prescription 6 months prior to the index date, had ≥ 1 inpatient or ≥ 2 outpatient SCD diagnoses and no diagnosis of other indications for hydroxyurea use between 6 months before and 1 year after the index date were included. Hydroxyurea adherence (Medication Possession Ratio; MPR) and the number of VOC events (emergency department visits with ICD-9 sickle crisis codes of 282.62, 282.64, 282.69 or 282.42) for 1 year following the index date, and covariates (age, gender, evidence of blood transfusion and a VOC event within 6 months before the index date) were measured. Descriptive statistics and logistic/Poisson regression analyses were performed.

RESULTS : A total of 592 patients (20.4±12.4 years) met the inclusion criteria. Of these, 15.5% were adherent to hydroxyurea (defined as MPR > 0.80) and mean MPR was 0.40 (± 0.30). In the 12 months following hydroxyurea initiation, 459 (77.5%) of the patients experienced ≥ 1 VOC event (mean=4.4 (± 8.9), median=1). After adjustment for covariates, being adherent to hydroxyurea was associated with a decreased risk of VOC events (Odds Ratio [OR]: 0.347, 95% CI: 0.207–0.582). Among patients who had ≥ 1 VOC event, Poisson regression showed that patients who were adherent to hydroxyurea were expected to have 66.7% fewer VOC events than those who were not, while controlling for covariates (Incidence Rate Ratio [IRR]:0.333, 95% CI: 0.267–0.415).

CONCLUSIONS : Being adherent to hydroxyurea among SCD patients is associated with a decreased risk and fewer number of VOC events.