Correlation between frequency of joint bleeds and FVIII activity levels. For each participant, the number of treated joint bleeds per 4-week interval was correlated with the median FVIII activity within each interval, as measured using the OS (blue circles) or CS (red circles) assay in BMN 270 clinical trials 270-201 (A) and 270-301 (B). The predicted bleed frequency per 4-week interval was modeled by negative binomial regression for OS activity (blue solid line) and CS activity (red dashed line). Shaded colored areas represent the corresponding 95% CI. No joint bleeds were reported for FVIII activity >50 IU/dL in either assay; these data were included in the modeling but are not shown here.