A 38-year-old woman presents with recurrent deep vein thrombosis (DVT) of the left lower limb and a spontaneous pulmonary embolism 6 months ago. She has no family history of thrombophilia and is not on oral contraceptives. Physical examination reveals mild splenomegaly. Routine coagulation studies (PT, aPTT, platelet count) are normal. Which investigation is most appropriate to establish the diagnosis?