Results of blinded quality control studied by 11 local participating laboratories. The International Society of Thrombosis and Haemostasis (ISTH)/Scientific Standardization Committee (SSC) standard plasma was aliquoted undiluted or diluted to 70%, 50%, 30%, or 20% using type 3 VWD plasma, and the type 2A and 2B plasmas were sent undiluted. All samples were blinded and sent to the 11 collaborating laboratories for assays of VWF:Ag and VWF activity (various methods) over a 2-year time period. The 2 top panels demonstrate the results of VWF:Ag and VWF activity from these laboratories. The bottom panel represents the specific activity of these results as calculated by VWF activity/VWF:Ag. On a 30% sample, the specific activity ranged between 0.22 and 0.91. There was also wide variation in the specific activity of type 2B VWD samples.