PB0660 - Impact of a product-specific reference standard for the measurement of a PEGylated rFVIII activity: an ex-vivo study.

Background: Monitoring activity of factor VIII (FVIII:C) for hemophiliac patients treated extended half-life products remains challenging. We previously demonstrated that, using FVIII-deficient plasma spiked with PEGylated rFVIII (Adynovi®), product-specific reference standards (SS) with a chromogenic assay (CA) was the most reliable method to evaluate FVIII:C.

Aims: The aim of this study was to compare different methods to evaluate FVIII:C in plasma samples of patients treated with PEGylated rFVIII (Adynovi®).

Methods: We used 73 samples of 8 patients treated by Adynovi® at different time points. We compared FVIII:C measured with one-stage clotting assay (OSA) or CA with local reference standard (LS) or SS and compared the results with CA and SS, our reference assay.

Results: Compared to our reference assay, FVIII:C levels correlated well using OSA with LS (r2=0.96, P< 0.001), OSA with SS (r2=0.96, P< 0.001) or CA with LS (r2=0.98, P< 0.001). When the results obtained with our reference assay were categorized into quartiles, there was only minimal differences between assays (Figure 1), with the largest mean difference in quartile #4 (-12%, p=0,01 with OSA with LS).

Conclusion(s): Unlike our previous study, this ex-vivo study shows a good correlation between the different FVIII:C assays. Although this study is based on a limited number of patients, it suggests that results of studies using spiked deficient plasma should be interpreted with caution.