The present invention provides a method for indicating von Willebrand factor deficiency in a blood sample or platelet-rich plasma sample which exhibits abnormal platelet function comprising (a) adding a von Willebrand factor preparation to the sample, and (b) testing the sample for platelet function. In this method, the von Willebrand factor preparation restores normal platelet function and indicates that the sample initially lacked the von Willebrand factor. The von Willebrand factor preparation comprises purified von Willebrand factor. The invention also discloses a method which further comprises separating the blood sample into a plasma layer and cellular component layer and removing the plasma layer, prior to the step of adding a von Willebrand factor preparation to the sample. In this embodiment the von Willebrand factor preparation comprises platelet-poor plasma containing a normal level of von Willebrand factor. The invention further provides a kit for indicating von Willebrand factor deficiency in a blood sample which exhibits abnormal platelet function comprising a von Willebrand factor preparation, wherein the preparation is added to the sample and restores normal platelet function, indicating that the sample initially lacked the von Willebrand factor.