Monitoring of von Willebrand factor inhibitors in patients with type 3 von Willebrand disease using a quantitative assay

Background Antibodies inhibiting von Willebrand factor (VWF) develop in a subset of patients with type 3 von Willebrand disease (VWD3) and may be detected by their inhibition of ristocetin cofactor activity (VWF:RCo). Some also inhibit factor VIII activity (VIII:C). Aim To describe monitoring of ten VWD3 patients for VWF inhibitors using a quantitative assay. Methods VWF inhibitor was measured by comparing VWF:RCo activity of a mix of patient and pooled normal plasma (PNP) with a mix of buffer and PNP, using agglutination of fixed normal platelets in microtiter plates or lyophilized platelets in an aggregometer. VIII:C inhibitor was measured by Bethesda assay. Preanalytical heat treatment of patient plasma was used during treatment episodes. Results Four of 10 patients monitored developed VWF inhibitors, two detected during bleeding episodes refractory to treatment and two on routine screening. Data from the first five patients were used to establish an arbitrary unit, VWU, defined as the amount of inhibitor per millilitre of patient plasma inactivating 25% of the activity of 1 mL of PNP. In three of four patients, both VWF:RCo and VIIII:C were inhibited at some time points, although VIII:C inhibition sometimes disappeared. In one patient, no VIII:C inhibition was seen. Two patients remained inhibitor positive more than 15 years after inhibitor detection, one became negative following immune tolerance induction, and one was deceased. Conclusions VWF inhibitors can be quantitatively monitored in VWD3 patients. Preanalytical heat treatment may be required for their detection post infusion.