Optimization of the automated, CS-2000i™ method for measuring low levels of von Willebrand factor ristocetin cofactor activity (VWF:RCo).

Measurements of von Willebrand factor ristocetin cofactor activity (VWF:RCo) are necessary for the diagnosis of von Willebrand disease (VWD). However, the conventional manual glass-plate method is technically demanding, and exhibits high intra-/inter-laboratory variation. An automated technique has been recently described utilizing the CS-2000i™ with specific VWF:RCo reagents, but measurements at lower levels (<10 IU/dL) lack reliability. We have optimized this automated system for measuring low levels of VWF:RCo. Using a sample volume (72 μL) and initial stirring speed (400 rpm), the lowest level of VWF:RCo detected with a CV < 20 % was 2.5 IU/dL. Greater variability was evident at this stirring speed with smaller sample volumes. Decreasing the stirring speed (200 rpm) with a sample volume (72 μL), however, significantly improved the CV, and enabled measurements at the lowest levels (≤2.5 IU/dL). The optimized assay demonstrated a high correlation (R(2) = 0.841) with the glass-plate method at <10 IU/dL VWF:RCo, suggesting that the different VWF:RCo reagents did not affect the results. In addition, changes in VWF:RCo post-DDAVP infusions were parallel in both assays, indicating that the automated method is suitable for the clinical monitoring of treatment in VWD. The optimized automated measurement of VWF:RCo offers a promising method for evaluating VWF at low levels of hemostatic activity.