Content and functional activity of von Willebrand factor in apheresis plasma

Von Willebrand Factor (VWF) is a complex high-molecular-weight (HMW) plasma glycoprotein playing a critical role in primary and secondary haemostasis. Owing to its multimeric structure and sensitivity to proteolysis, VWF can be used as a marker of the impact of collection procedures on the characteristics of plasma for transfusion and for fractionation. We studied VWF content, functional activity and HMW multimers in plasmas collected by five different automated apheresis collection procedures.

Five series of 30 plasma units were obtained from volunteer donors at two collection sites using Haemonetics PCS2 machines with Revision (Rev) F, Rev G, high-separation core (HSC), or filter core (FC) procedures, or Baxter-Fenwall Autopheresis-C (Auto-C). VWF antigen (VWF:Ag), ristocetin cofactor (VWF:RCo) activity and HMW multimers were first determined in 10 randomly selected plasma donations collected with Rev G, HSC, FC and Auto-C procedures. Then, the same analyses and the collagen binding (VWF:CB) activity were determined in the pools of 30 donations from each of the five procedures and compared with two normal plasma pools (NPP1 and NPP2). A reference plasma (RP) was used to calibrate each assay.

There were a greater number of group O individuals in the Rev F, Rev G and FC donors than in the HSC and Auto-C donors. The mean VWF:Ag level was > 100 IU/dl, VWF:RCo activity was > 90 U/dl, the VWF:RCo/Ag ratio was close to 1, and the percentage of 11-15 mers was above 100% of RP in the 10 individual plasma units from Rev G, HSC, FC, and Auto-C and in their respective pools. The mean percentage of multimers > 15 mers, relative to RP, was significantly less in Rev G plasmas (48 +/- 17%; range 32-91%), compared with Auto-C, HSC and FC plasmas (P = 0.0211; 0.0257; and 0.0376, respectively). The VWF:CB activity of the 30-donation pools was 61 and 60 U/dl in Auto C and HSC, 50 U/dl in Rev F and FC, and 43 U/dl in the Rev G pool. The VWF:CB/Ag ratio was 0.54 (Auto-C), 0.49 (HSC), 0.46 (Rev F), 0.45 (FC) and 0.37 (Rev G), compared with 0.81and 0.92 in NPPs. The percentage of VWF multimers of 11-15 mers in apheresis plasma and NPP was normal. VWF multimers > 15 mers ranged from 38 to 64% of that of RP plasma, and was 111 and 112% in NPPs.

The VWF:Ag, VWF:RCo activity and 11-15 mer VWF multimers were well preserved in all plasma units from each of the five apheresis procedures. The VWF:CB activity and the percentage of multimers > 15 mers in apheresis plasma was less than in normal plasma pools and differed slightly among procedures.