Figure 2. Age, fetal hemoglobin, and pain crises. Relationships between the age groups, mean HbF, and APC rate: at about 10-12 years of age and older, HbF generally drops to under 10%, which coincides with significantly increased APC. Numbers of patients from left to right in different age groups were: n=19 (1-4.9 years); n=12 (5-9.9 years); n=6 (10-14.9 years), n=7 (15-19.9 years), and n=6 (≥20 years). SEM: Standard error of mean; APC: annual pain crisis; HbF: fetal hemoglobin.