VWF and von Willebrand disease (hereditary or acquired): Initial laboratory studies include complete blood count (CBC) with platelet counts and morphology, prothrombin time (PT), activated partial thromboplastin time (aPTT) and fibrinogen or thrombin and VWF tests for VWD diagnosis [ristocetin cofactor (VWF:RCo), VWF antigen (VWF:Ag), VWF:RCo / VWF:Ag ratios and factor VIII activity (F VIII:C)].