Mean predose FIX activity during trial. The graph shows the mean (± standard error of the mean) FIX activity in predose samples for the 10 IU/kg and 40 IU/kg prophylaxis groups. A 1-stage clotting assay was used to determine the FIX activity (IU/dL) in predose blood samples collected at clinical visits during the trial. Analysis was performed at a central laboratory, using a product-specific reference standard for calculation of the FIX activity.