The invention relates to a diagnostic method for determining the von Willebrand factor (VWF) cleaving activity of ADAMTS-13 in a test medium during which the test medium is mixed with 0.5 to 5 U/ml of a von Willebrand factor (VWF) that does not contain ADAMTS-13, and after incubation, the ADAMTS-13 activity is determined based on the drop in the VWF-mediated aggregation of thrombocytes.