Is low ADAMTS-13 activity a valid criterion to distinguish between TTP and HUS?
Taking into account the differences in pathophysiology, one would assume that TTP could be easily separated from HUS by its low or absent activity of ADAMTS-13. This issue, however, is not as straightforward as it sounds and remains highly controversial [27 34]. Some studies demonstrated that severely deficient ADAMTS-13 activity is specific for TTP [27,28]. For example, Bianchi et al. reported that severe deficiency of ADAMTS-13 activity (<5%) is specific for TTP, whereas less severe reductions (between 30 and 10%) are occasionally found in other thrombocytopenic states as well, including severe sepsis, myelodysplastic syndrome and heparin-induced thrombocytopenia type 2 [27]. In another study, Tsai and colleagues found normal ADAMTS-13 activity in 16 patients with D+ HUS [28]. In fact, patients with HUS had a decrease in UL-vWF, presumably caused by enhanced proteolysis resulting from abnormal shear stress in the microcirculation [28]. ADAMTS-13 activity was normal in 29 children wit
