Careful selection of sample dilution and factor-V-deficient plasma makes the modified activated protein C resistance test highly specific for the factor V Leiden mutation

The aim of this study was to evaluate critically the recently modified acti vated-partial-thromboplastin-time (APTT)-based activated protein C (APC)-re sistance tests, which are more specific for the factor V Leiden mutation th an the first generation APC-resistance tests. The only modification to thes e tests is the predilution of the plasma sample in factor-V-deficient plasm a. The intended effect of this predilution is to bring the concentrations o f all clotting factors, except factor V, to the same normal levels. This, i n principle, makes the tests also suitable for assaying the plasma of patie nts treated with oral anticoagulants and heparin, or of patients with a lup us anticoagulant. However, not every factor-V-deficient plasma is suitable for this application Because the factor V:factor VIII ratio is important in establishing the APC ratio, the factor-V-deficient plasma should contain a sufficiently high factor VIII concentration. We also found that the optima l dilution to obtain the same APC ratios for patients, whether or not treat ed with coumarins or heparin, is not the same for each test or factor-V-def icient plasma. We compared two modified APTT-based APC-resistance tests (on e developed in our laboratory and one commercial) with respect to their abi lity to discriminate between carriers and non-carriers of the factor V Leid en mutation. Both modified tests gave complete separation of carriers and n on-carriers of the factor V Leiden mutation whether or not they are treated with anticoagulants. This makes these tests very suitable for routine scre ening. (C) 1999 Lippincott Williams & Wilkins.