A 45 year old lady presents with normal PT and increased aPTT. About 2 years ago, she was operated for cholecystectomy & did not have any bleeding episode. What is next investigation for clinical diagnosis?

Choice A is ruled out as hemophilia presents in males and presents with bleeding Choice C is ruled out as platelet aggregation test is used for platelet disorders which lead to bleeding. Ristocetin co-factor assays are used to detect Von Willebrand's disease. Von Willebrand's disease causes prolongation of PTT but it is also associated with clinical bleeding so choice D can be ruled out. An isolated result showing aPTT prolongation suggests a deficiency or inhibitor of one or more of the intrinsic pathway clotting factors Russel viper venom test (dRVVT) Dilute Russel viper venom test is one of the tests to detect lupus anticoagulant. Lupus anticoagulant is associated with prolongation in PTT and thrombosis but no bleeding.