A Practical Guide to Haemostasis

Russell Viper Venom [RVV] Factor X Assay


There are five assays for Factor X:
   - 1-stage PT-based Assay
   - 1-stage APTT-based Assay
   - Chromogenic Factor X Assay
   - Immunological Factor X Assay
   - Russell's Viper Venom Assay

Russell's Viper Venom (RVV) activates Factor X directly and in the presence of Factor V, Prothrombin [Factor II], Calcium and Phospholipid results in a fibrin clot. RVV is isolated from the snake - Daboia russelii.


The principle of the RVV Factor X assay is similar to that of the 1-stage PT-based Factor Assay.  A series of dilutions of the reference and test plasma are made and an aliquot of the Factor X deficient substrate plasma is mixed with an aliquot of the diluted reference or test plasma, incubated at 37°C for 30s and then the RVV-platelet substitute is added. 30s later, clotting is initiated by the addition of 0.025M Calcium Chloride and the time to clot formation recorded The clotting times are recorded, plotted on Log-Log paper and by comparison to the reference Factor X standard the concentration of FX in the test plasma can be established.

Factor X  Reference or test plasma Dilutions: 1/10, 1/20, 1/140, 1/100
Russell Viper Venom (RVV) + Platelet substitute Purchased commercially
0.025M Calcium Chloride  
Factor X deficient substrate plasma Purchased commercially


Low Factor X levels may be seen in the following:

Causes of a Low Factor X Assay
In association with other clotting factor deficiencies in a patient with vitamin K deficiency or in patients on vitamin K antagonists e.g. Warfarin
Congenital Factor X deficiency
Liver disease
Amyloidosis due to adsorption of Factor X onto the Amyloid fibrils
Acquired Factor X inhibitors [rare]
Patients with deletions of chromosome 13 [chr13q] may have combined Factor X and Factor VII deficiency as the genes for these two proteins lie close together on the long arm of chromosome 13.
A deficiency in one of the enzymes involved in Vitamin K metabolism e.g. VKORC1

Reference Ranges

The reference ranges are identical to that of the PT-based FX assay.

What Test Next?

See Interpretation of Results - which suggests what test(s) to do next.