A single point mutation of the
factor V (FV) gene, leading to the substitution Arg506Gln in the FV molecule (
FV-Leiden) and hence resistance to its breakdown by activated
protein C (APC), is the most prevalent risk factor for
venous thrombosis in the Caucasians. A ratio determined by activated partial thromboplastin time (APTT) of test plasma in the presence or absence of exogenous APC (the APC ratio), is the method widely used to screen individuals with this risk factor for
thrombosis. Because of functional defects of
vitamin K-dependent
clotting factors in patients on oral
anticoagulant therapy, this method cannot be applied to those patients without modification. One modification is to mix test plasma (1:5 or 1:10) with FV-deficient plasma so that 80-90% of functioning
vitamin K-dependent factors are supplied by the FV-deficient plasma. Even with 10-20% of FV in the mixture,
APC-resistance still can be demonstrated. In this report, we present our results of the modified APC-sensitivity assay using FV-deficient plasma from different commercial sources. APC ratios determined by the original method in which test plasma is not mixed with FV-deficient plasma can be significantly different from those determined by the modified method in which test plasma is diluted 1:5 with FV-deficient plasma. This difference between methods was observed not only in normal individuals, but also in
FV-Leiden positive individuals, and in patients on
warfarin therapy. Further, APC ratios varied significantly depending on the commercial source of the FV-deficient plasma. The modified method is apparently suitable to identify
APC-resistance in patients on
warfarin therapy, as well as in individuals not receiving
anticoagulant treatment. However, one must be aware that
APC-resistance ratios obtained with the modified method are likely to be different from those established with the original method, and the source of FV-deficient plasma can be
a factor influencing the ratios in the former cases.