Inherited resistance to activated protein C (APCr) is currently recognized as the most prevalent cause underlying venous thrombophilia, with an estimated prevalence around 20% in thrombotic patients and around 1.8-7% in the general population. A correct laboratory diagnosis of APCr is therefore essential. Two different diagnostic approaches are at present at our disposal: the semi-quantitative plasma test based on the measurement of two aPTTs (in the presence and absence of activated protein C), and the detection of the factor V Arg506 Gln mutation by DNA analysis. In this study we firstly evaluated sensitivity, specificity and diagnostic efficiency of an aPTT-based plasma clotting test (Chromogenix, Sweden) versus DNA analysis; then, since the APC resistance test is invalidated by a basally prolonged aPTT (i.e. during warfarin and heparin therapy or in patients with clotting factor deficiencies or in the presence of a lupus anticoagulant), patient plasmas were conveniently diluted in factor V deficient plasma in order to correct clotting factor abnormalities. Nevertheless, patients with a LA and an aPTT ratio range 1.8-3.17 were still all misclassified. We obtained correct diagnoses in LA positive patients by preincubating plasmas with a mixture of phospholipids; therefore we decided to perform a double modified clotting test adding a mixture of platelet derived phospholipids to samples previously diluted in factor V deficient plasma. The performance characteristics of this novel method with a different aPTT reagent (Behring, Germany) were also evaluated. With this double modified test all patients were correctly classified as negative or positive for factor V mutation in agreement with DNA analysis, irrespectfully of the basal aPTT value and the aPTT reagent employed. We propose this modified version of the APCr clotting test as an easily reproducible, reliable, very sensitive and specific screening test which possibly reduces the need for DNA analysis.