Novel methods are described for measuring the rate of ADAMTS13-mediated cleavage of von Willebrand Factor (VWF) multimers. Through the use of the reagent, ristocetin, the method can advantageously be performed in the absence of an applied shear stress or added denaturing agent. Also described are methods for diagnosing ADAMTS13-associated disorders and methods for providing improved treatment of those disorders by evaluating the efficacy of the treatment using the methods as described.