A Practical Guide to Laboratory Haemostasis


1-Stage Russell Viper Venom [RVV] Factor X Assay


Russell's Viper Venom (RVV) activates factor X directly and in the presence of Factor V, prothrombin, Calcium and Phospholipid results in a fibrin clot. RVV is isolated from Daboia russelii.
The RVV assay is one of 5 assays for measuring factor X [the 4 others are: a PT-based assay, an APTT-based assay, a chromogenic assay and an immunological assay] although the PT and APTT-based assays are the most commonly performed.


The principle of the RVV Factor X 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 clotted with RVV (see below). The clotting times are recorded, plotted on Log-Log paper and by comparison to the reference FX standard the concentration of FX in the test plasma can be established.


Reference or test plasma Dilutions: 1/10, 1/20, 1/140, 1/100
RVV + Platelet substitute Purchased commercially
0.025M Calcium Chloride  
FX deficient substrate plasma Purchased commercially

An aliquot of the FX 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 results are plotted on Log-Log paper and the results derived as for the 1-stage PT-based assay.


Low Factor X levels may be seen in the following:

Causes of a low factor X
In association with other clotting factor deficiencies in vitamin K deficiency and in patients on vitamin K antagonists e.g. warfarin
Congenital Factor X deficiency
Liver disease
Amyloidosis due to absorption of FX onto the amyloid fibrils
Acquired factor inhibitors [rare]
Patients with deletions of chromosome 13 [chr13q] may have combined FX and FVII deficiency as the genes for these two proteins lie close together on the long arm of chromosome 13.

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.

Data Interpretation

Click HERE to go to the Data Interpretation Exercises.