A rapid assay for ristocetin cofactor activity using an automated coagulometer (ACL 9000)

We have set up a rapid assay for measuring the activity of the von Willebrand factor ristocetin cofactor (VWF : RCo) using an automated coagulometer (ACL 9000; Instrumentation Laboratory, Lexington, Massachusetts, USA) and commercially available lyophilized platelet reagents (Dade-Behring, Marburg, Germany). Since VWF : RCo tested with the coagulometer (ACL-VWF : RCo) did not require loading, calculation of von Willebrand factor (VWF) activity or pretreatment of samples (calibration standard and tested plasmas), we thought it might be useful for rapid, automatic screening of VWF activity. To assess the precision and the potency of this ACL-VWF : RCo, we tested the assay in this laboratory's internal normal and abnormal controls, in a group of 67 healthy individuals and in 28 patients with different types of von Willebrand disease (VWD). We compared the ACL-VWF : RCo findings with those of the standard agglutination test (Agg-VWF : RCo), normalizing both assays against VWF antigen (VWF : Ag) measured by 'automatic' and standard enzyme-linked immunosorbent assay 'in-house' methods, to calculate the VWF : RCo/VWF Ag ratios. The within-assay and between-assay repeatability of the automatic ACL-VWF : RCo gave coefficients of variation < 5%, and reproducibility in normal and abnormal laboratory controls gave coefficients of variation of 7.9 and 9.3%, respectively. In VWD patients the results were equivalent to those of the standard Agg-VWF : RCo assay but, because of the good sensitivity, the ACL-VWF : RCo was more accurate in evaluating the VWF : RCo and establishing the VWF : RCo/VWF : Ag ratios in VWD patients with low VWF levels. Moreover, this ACL-VWF : RCo is faster than Agg-VWF : RCo, providing results of calibration curves and of 10 patient samples within 15 min. We can conclude that this automatic ACL-VWF : RCo gives a reliable and useful measure of VWF activity and can be proposed as a screening test for rapid diagnosis of VWD even in patients with low VWF levels in plasma.