Patient data handling
In both cohorts FIX activity was measured using the one-stage assay (OSA) according to local protocol. Laboratory specifications of all participating sites are shown in Supplementary Table 1. All FIX activity levels measured during bleeding episodes or surgeries18 were excluded from this analysis.
Since the OSA does not distinguish FIX activity from the respective FIX concentrates (e.g. residual FIX activity levels from the previous FIX dose or currently present FIX concentrate), it is required to correct for previously administered factor concentrates. To do so, we performed the following corrections in line with Diao et al.16and previously reported PK analyses with FIX concentrates9,12,19,20:
\(\left(1\right)\ Residual\ decay\ correction=\left(Predose\ activity-baseline\right)*e^{-kt}\)
\(\left(2\right)\ Corrected\ FIX\ activity=Measured\ FIX\ activity-baseline-residual\ decay\ correction\)
\(\left(3\right)\ k=\frac{\ln\left(2\right)}{t_{1/2}}\);
in which k represents the elimination rate constant of the previously used concentrate for rFIX-Fc (Alprolix®), rFIX (Benefix®) or rIX-FP (Idelvion®) calculated for each age group (<6, ≥6-12, ≥12-18 and ≥18 years). For these calculations, we used typical half-lives (t1/2) of each age group as reported by the respective European Public Assessment Report (EPAR)21–23. Other patient characteristics collected were age, height, body weight, lean body weight (LBW) and fat free mass (FFM). Occasions were defined as a visit with PK assessment, as described in literature24.