Comparison of pdR338L-FIX expressed by FLT180a-treated patients and rR338L-FIX. (A) R338L-FIX:C in plasma samples collected from FLT180a-treated patients was measured using both the SynthASil OSA and ROX FIX CSA methods. The resulting values were compared with the activity of rR338L-FIX spiked in hemophilia B plasma at 5, 20, 50, 100, and 150 IU/dL FIX:C, determined by the SynthASil OSA FIX method. (B) Empirical SynthASil OSA FIX:C results were used in conjunction with FIX:C ratios (derived from supplemental Table 2) to estimate ROX FIX:C results, which are directly compared with empirical ROX FIX:C values. FS, field study.