A method for determining an appropriate treatment option for a patient who has been diagnosed with disseminated intravascular coagulation (DIC) but who may have thrombotic thrombocytopenic purpura (TTP), by analyzing the amount and/or enzyme activity of a von Willebrand factor (vWF)-cleaving protease (ADAMTS13) and the amount of vWF in a patient that has been diagnosed with DIC is disclosed. Using the method of the present invention, a differential diagnosis of patients with thrombotic thrombocytopenic purpura (TTP) can be made from among patients diagnosed with DIC, which could not previously be distinguished on the basis of only clinical findings or known markers. Also disclosed is a kit for determining an appropriate treatment option, the kit comprising an antibody or a fragment thereof which specifically binds to ADAMTS13.