Comparison of several von Willebrand factor (VWF) activity assays for monitoring patients undergoing treatment with VWF/FVIII concentrates: improved performance with a new modified automated method.

BACKGROUND: The ability of von Willebrand factor (VWF) to bind platelet GP Ib and promote platelet plug formation is measured in vitro using the ristocetin cofactor (VWF:RCo) assay. Automated assay systems make testing more accessible for diagnosis, but do not necessarily improve sensitivity and accuracy. OBJECTIVE: We assessed the performance of a modified automated VWF:RCo assay protocol for the Behring Coagulation System (BCS(®) ) compared to other available assay methods. METHODS: Results from different VWF:RCo assays in a number of specialized commercial and research testing laboratories were compared using plasma samples with varying VWF:RCo activities (0-1.2 IU mL(-1) ). Samples were prepared by mixing VWF concentrate or plasma standard into VWF-depleted plasma. Commercially available lyophilized standard human plasma was also studied. Emphasis was put on the low measuring range. VWF:RCo accuracy was calculated based on the expected values, whereas precision was obtained from repeated measurements. RESULTS: In the physiological concentration range, most of the automated tests resulted in acceptable accuracy, with varying reproducibility dependent on the method. However, several assays were inaccurate in the low measuring range. Only the modified BCS protocol showed acceptable accuracy over the entire measuring range with improved reproducibility. CONCLUSIONS: A modified BCS(®) VWF:RCo method can improve sensitivity and thus enhances the measuring range. Furthermore, the modified BCS(®) assay displayed good precision. This study indicates that the specific modifications - namely the combination of increased ristocetin concentration, reduced platelet content, VWF-depleted plasma as on-board diluent and a two-curve calculation mode - reduces the issues seen with current VWF:RCo activity assays.

Pitfalls in special coagulation testing: three illustrative case studies.

Assays in the special coagulation laboratory are affected by numerous factors, including pre-analytical variables, anticoagulant drugs, and abnormalities of the coagulation system other than the analyte specifically being examined. By reviewing special coagulation tests as a group and in concert with clinical information, as well as understanding assay methodologies, interferences can be more easily recognized and incorrect interpretations avoided, preventing possibly unnecessary treatment of patients. Three case studies involving protein S activity, von Willebrand factor analysis, and factor V activity with Bethesda titer will highlight potential pitfalls in the interpretation of special coagulation tests.