PB1107 - A specific threshold for ELISA detection of Von Willebrand alloantibodies in patients diagnosed as type 3 von Willebrand disease.

Background: The development of anti-VWF antibodies can occur in two clinical situations: alloantibodies in patients with type 3 von Willebrand disease (VWD) and autoantibodies in several underlying conditions (e.g. lymphoproliferative syndrome) leading to acquired von Willebrand syndrome (AVWS. Both types of antibodies can be detected by an ELISA.
The parameters of the home-made ELISA for the detection of anti-VWF antibodies in our laboratory have recently been modified. Plasma-derived Von Willebrand factor (pVWF), previously used to coat the microplates, has been replaced by vonicog alfa, a recombinant VWF (rVWF
Results are expressed as optical density (OD). The threshold of positivity, defined as mean + 2 standard deviation (SD), was calculated on 42 healthy donors. A signal is positive if the OD is greater than 0.230. Using this threshold, detection of anti-VWF antibodies was slightly positive in a proportion of type 3 VWD patients, whereas it was previously negative with plasma VWF ELISA.

Aims: To evaluate the threshold of positivity for antibody detection in type 3 VWD patients

Methods: ELISA with rVWF was realized in a total of 41 samples from 20 patients (without any replacement therapy) diagnosed as type 3 VWD and well characterized. Briefly, rVWF is coated in microplates. After overnight at room temperature, the samples were dispensed and antibody was revealed by an anti-IGg1 HRP. Mean, SD and threshold were calculated using the 41 values obtained.

Results: Mean and SD were respectively 0.237 and 0.12. A threshold of positivity of 0.470 was obtained (mean+ 2 SD) for patients with type 3 VWD, which is higher than that found in the healthy population.

Conclusion(s): A specific threshold of positivity was characterized and must be used for alloantibody detection in type 3 VWD patients. Thus, when prescribing an anti-Willebrand antibody test, the context should be clarified in order to use the appropriate positivity threshold.