A 58-year-old woman with acute coronary syndrome is treated with dual antiplatelet therapy (aspirin + clopidogrel). After 3 weeks, she develops petechiae, epistaxis, and gingival bleeding. Her hemoglobin is 9.2 g/dL (previously 13.5 g/dL). Which investigation would best establish the diagnosis of clopidogrel-induced thrombotic thrombocytopenic purpura (TTP)?