Scientific researcher Good Biomarker Sciences Leiden, Zuid-Holland, Netherlands
Background: Factor VIII circulates in a reversible complex with von Willebrand factor (VWF), usually complexed for >95%. It can be calculated that this complex remains highly associated up to very low factor VIII levels. It is secured as complex in the One-stage Clotting Assay (OCA) by a final dilution before assay of the sample 1:1 with vWF containing factor VIII deficient plasma. Consequently, it is the activity of the complex that is assayed in the OCA.
Aims: .
Methods: .
Results: For normal plasma FVIII and Advate, clotting activity of free and complexed FVIII is similar. However, for modified FVIII (Refacto, Eloctate, Adynovi) added VWF (Wilfactin) inhibits their coagulation activities. For Eloctate and Refacto this concerns a maximal inhibition of 74 and 52%. Adynovi showed maximal inhibition of 27%. The optimal inhibitory effect is reached at 50% of added VWF. Thus, also for these preparations in the OCA the activity of the complex will be recorded. This is different for the chromogenic assay (CA) which records factor VIII independent of VWF. Comparing five preparations with labeled activity in the OCA and CA, both calibrated with normal plasma, showed especially for Eloctate a discrepancy with 0,44 x lower activity in the OCA as expected from VWF inhibition. For Advate and Beriate activities were similar in both assays. In a VWF free APTT spiking of Eloctata and Advate activities were similar as expected. The labeling of the five preparations showed for three agreements with our analysis with CA, but was fourfold lower for Advate and Beriate.
Conclusion(s): Normal factor VIII can be measured and labeled with both CA and OCA; our results indicate that the preferred method for modified FVIII is the OCA. In patients treated with modified FVIII, the OCA method should be used as pharmacodynamic method. The labeling of preparations needs to be reconsidered.
