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|
|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.|
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.
Click HERE to go to the Data Interpretation Exercises.