The dilute Russell viper venom test is a snake venom that contains a factor X activating enzyme and a factor V (FV) activator, which is strongly phospholipid-dependent. The screening reagent contains Russell viper venom (RVV), calcium ions, and phospholipids. The venom activates the coagulation cascade at factor X and activates factors below factor X (I, II, and V), eliminating any influence of factors above factor X in the coagulation cascade (VIII, IX, XI, and XII). As a result, the test is not affected by abnormalities in the contact factors or deficiencies or inhibitors in factors VIII and IX.
The first part of the assay is a dRVVT screening test which uses a reagent with a low concentration of phospholipids. When there are autoantibodies against phospholipids in the sample, they are partly neutralized, resulting in a prolonged clotting time. The confirmatory test uses dRVVT with a higher concentration of phospholipids resulting in a shortened clotting time. The final result is a ratio of the screen and confirmatory. If the ratio is > 1.2, the result is positive.
Guidelines have suggested performing a normalized ratio and a ratio to eliminate any false positives due to a factor deficiency.
Antibodies to FV or a deficiency of FV will prolong the dRVVT; however, the prolongation will not correct when a correction is performed. This is done by calculating a corrected ratio using pooled normal plasma (PNP) - dRVVT screen is divided by the dRVVT screen of the PNP, as is the dRVVT confirmatory is divided by the dRVVT confirmation of the PNP.
In patients on warfarin, the dRVVT is unreliable. Laboratories can perform a 1:1 mix with PNP to correct for the presence of warfarin.
Heparin will also prolong the dRVVT, but this can be detected within the prolongation of the thrombin time. It is important to perform this test to ensure the sample does not contain heparin which may result in a false positive test.