Dilute Russell's Viper Venom Time - Mechanism

Mechanism

This in vitro diagnostic test is based on the ability of the venom of the Russell's viper to induce thrombosis. The coagulant in the venom directly activates factor X, which turns prothrombin into thrombin in the presence of factor V and phospholipid. In the dRVVT assay, low, rate-limiting concentrations of both Russell's viper venom and phospholipid are used to give a standard clotting time of 23 to 27 seconds. This makes the test sensitive to the presence of lupus anticoagulants, because these antibodies interfere with the clot-promoting role of phospholipid in vitro, and their presence results in a prolonged clotting time. A mixing study is then performed, which consists of adding an equal volume of the patient's plasma to normal plasma. In this study, one would expect the clotting time to return to the normal range if there was only a deficiency of coagulation factors alone. A prolonged clotting time of 30 seconds or greater that does not correct despite the mixing study suggests the presence of a lupus anticoagulant. An abnormal result for the initial dRVVT assay should be followed by a dRVVT confirmatory test. In this test, the inhibitory effect of lupus anticoagulants on phospholipids in the dRVVT can be overcome by adding an excess of phospholipid to the assay. The clotting times of both the initial dRVVT assay and confirmatory test are normalized and then used to determine a ratio of time without phospholipid excess to time with phospholipid excess. In general, a ratio of greater than 1.2 is considered a positive result and implies that the patient may have antiphospholipid antibodies. The dRVVT test is more sensitive than the aPTT test for the detection of lupus anticoagulant, because it is not influenced by deficiencies or inhibitors of clotting factors VIII, IX or XI.