Willebrand factor ristocetin cofactor activity assay using a novel automated assay protocol. J Thromb Haemost 2010; 8: 2216-23.Summary. Background, objectives and methods: An accurate, sensitive and precise assay for reliable determination of the ristocetin cofactor activity of von Willebrand factor (VWF:RCo) in plasma and von Willebrand Factor (VWF)-containing concentrates has been evaluated. The assay is based on a commercially available automated protocol with modifications including a combination of adding additional ristocetin and the use of two calibration curves for the high and low measuring ranges. Results: Addition of extra ristocetin resulted in improved measurement of VWF recoveries from various VWF-containing concentrates that were underestimated using the standard automated protocol. The modifications resulted in improved assay performance over an extended measuring range (2.00-0.03 IU mL )1). Accuracy was tested using VWF deficiency plasma spiked with the 1st international standard (IS) for VWF concentrate. Seven dilutions, ranging from 1.80 to 0.05 IU mL )1, were analyzed and resulted in measured concentrations between 80% and 100% of the assigned potency of the standard. Linearity was determined from the regression plot of the same concentrate dilutions and resulted in a correlation coefficient of 0.998. The repeatability, expressed as coefficient of variation, was 2% in the normal range (0.90 IU mL . The corresponding reproducibility results were 2% and 15% at the normal and low measuring ranges, respectively. Conclusions: Analysis of patients with von Willebrand disease (VWD) indicates that the modified automated BCS Ò protocol has a superior discrimination power compared with the standard protocol. This is especially true in samples with low VWF, as in patients with type 3 VWD.