Low ADAMTS13 activity is associated with an increased risk of thrombotic thrombocytopenic purpura (TTP). This may be caused by the presence of inhibitory autoantibody (immune-mediated TTP) or genetic deficiency of the enzyme. The latter condition is also known as Upshaw-Schulman syndrome (USS), the extremely rare congenital form of TTP. The clinical picture typically presents during infancy or early childhood but an increasing number of late-onset cases are detected.