Protease activity of vWF in samples from patients with various thrombocytopenic disorders and control samples.

The vWF protease activity in samples from patients with various thrombocytopenic disorders, healthy controls, and hospitalized patients without thrombocytopenia was measured using previously described methods. Most of the samples from healthy controls and hospital controls with normal platelet counts had normal vWF protease activity (3+ to 4+); however, some samples had decreased vWF protease activity (1+ to 2+). Many of the TTP samples had decreased vWF protease activity, but there were also TTP samples with normal vWF protease activity. The other thrombocytopenic groups also had samples with decreased vWF protease activity.