Chromogenic Factor X Assay

Patients with a Lupus Anticoagulant often exhibit variable prolongation of phospholipid-dependent clotting tests. In some cases, the Lupus Anticoagulant can interfere with the Prothrombin Time used to monitor coumadin therapy. This can significantly complicate the control of oral anticoagulant therapy in such patients.

The Hemostasis and Thrombosis Laboratory offers the chromogenic assay of Factor X to determine whether a patient with a Lupus Anticoagulant is therapeutically anticoagulated. The chromogenic assay is not dependent on phospholipid and is therefore unaffected by the presence of the Lupus Anticoagulant.

The method is based on a two-stage principle. In stage 1, Factor X is activated in the presence of calcium ions to factor Xa (FXa) by Russell's Viper Venom (RVV). In stage 2, the FXa generated hydrolyses the chromogenic substrate S-2765, thus liberating the chromophoric group p-nitroanaline (pNA). The color is read photometrically at 405 nm. The amount of FXa generated, and hence the intensity of the color formed, is proportional to the plasma Factor X concentration.

Because Factor X levels take several days to fall, the Chromogenic FX assay can only be used to monitor patients after the initiation phase of coumadin treatment is complete. The therapeutic range for FX activity is 15 - 30%.