Measuring platelet receptor affinity for von Willebrand factor The ristocetin aggregation test is a bioassay for activity. Ristocetin, a tetracycline antibiotic, binds to platelets in vitro and activates vWF receptors. This antibiotic binding leads to platelet aggregation if vWF is available to serve as a bridge between platelets. The platelet aggregation can be measured optically, or by other means.