A 35 year old lady presents with an isolated prolongation of aPTT. Prothrombin time (PT) and platelet count are normal and there is no obvious bleeding tendency. Two years back, she was operated for cholecystectomy and had no adverse bleeding episode. Which of the following should be the next step in evaluating this patient

Answer is C (Russell viper venom assay): 'If there is no bleeding history, but the APTT alone is prolonged, the most likely cause of prolonged aPTT is a lupus anticoagulant' - 'Complications in Surgery' by Dohey (2006)/187 Russell Viper Venom assay is done to establish the presence of lupus anticoagulant and is the single best answer amongst the options provided. Isolated prolongation of aPTT with no clinical history of bleeding suggests the presence of Lupus anticoagulant or deficiency of contact factors (Factor XII, Prekallikrein or HMWK) Factor VIII deficiency, Defect in platelet aggregation and vWF disease all lead to variable amount of clinical bleeding. No options are provided to detect deficiency of contact factors in this question and hence Russell Viper Venom assay is the answer of choice here