A 42-year-old man with a 10-year history of recurrent thrombosis (3 DVTs, 2 PEs) is found to have a prolonged activated partial thromboplastin time (aPTT) that does not correct on mixing study. Protein C, protein S, antithrombin, and factor assays are normal. Which single laboratory finding best distinguishes antiphospholipid syndrome (APS) from factor deficiencies in this patient?