Resistance to activated
protein C (APC) is the most common defect found in patients who have
venous thromboembolism. The molecular basis of
APC resistance is a single-point mutation (arginine506-glutamine) in the gene that encodes for
coagulation factor V. This mutation results in
a factor V molecule (
factor V(Leiden)) that is less effectively downregulated by APC than is normal
factor V. The gold standard for the detection of this defect is
DNA analysis. Several functional tests, which are based on activated partial thromboplastin time clotting assays, are also commercially available for the detection of
APC resistance. These tests, however, have not been satisfactory. Compared with the results of
DNA analysis, the results of these tests are frequently discordant. Further, in some patients (eg, those who have
lupus anticoagulant or have been receiving
heparin), these tests cannot be performed at all.
DNA analysis is therefore required in most patients to distinguish congenital
APC resistance (
factor V(Leiden)) from other causes of abnormal response in functional
APC-resistance tests. The purpose of this study was to investigate the clinical use of a new chromogenic
APC-resistance assay that is based on direct measurement of the effect of APC on
factor Va cofactor activity in highly diluted,
thrombin-activated plasma specimens. All individuals who provided plasma samples for the study underwent
DNA analysis to detect the presence of the
factor V mutation. In all patient subgroups, including patients who had
lupus anticoagulant and those who were receiving
unfractionated heparin or
coumarin derivatives, the chromogenic test showed excellent discrimination between normal individuals and those who were heterozygous or homozygous for the
factor V(Leiden) mutation. No discordant results with
DNA analysis were found in 150 cases. The new test easily can be incorporated in any laboratory that has an automated coagulation apparatus with an option for chromogenic measurements. All
reagents are commercially available at low cost, and the test is easy to perform and is not time-consuming. This new, sensitive, and specific test allows large-scale screening for the
factor V(Leiden) mutation without the need for
DNA analysis.