Figure 4. HU adherence versus MCV, HbF, age, and APC. Corresponding panels A-D show the difference in MCV, HbF, age and APC based on HU adherence (50-80% adherence vs. 90-100% adherence). Statistically significant difference (based on t-test) between the HU adherence groups exists only for MCV, which is higher with better hydroxyurea adherence. Box plots with error bars represent means and standard deviations. APC: Annual pain crisis; HbF: fetal hemoglobin; MCV: mean corpuscular volume; HU: hydroxyurea.