Definitions

russell's viper venom

Dilute Russell's viper venom time

Dilute Russell's viper venom time (dRVVT) is a laboratory test often used for detection of lupus anticoagulant (LA). 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 clotting time of 23 to 27 seconds. This makes the test sensitive to the presence of lupus anticoagulants, since these antibodies interfere with the clot-promoting role of phospholipid in vitro. The inhibitory effect of LA on dRVVT can be overcome by adding an excess of phospholipid to the assay. These two clotting times are normalized and then used to determine a ratio of time without phospholipid to time with phospholipid. In general, a ratio of greater than 1.2 is considered a positive result and implies that the patient may have antiphospolipid antibodies. However, before diagnosis other test should be performed, such as mixing studies which rule out the possibility that clot prolongation is due to an inhibitor. 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.

The dRVVT is one component of a workup of a suspected antiphospholipid antibody, the other component being the serological testing for anticardiolipin antibodies using ELISA technology. The Sapporo criteria require both a positive dRVVT and serological testing, in the setting of clinical thrombosis or recurrent fetal loss, to diagnose an antiphospholipid antibody syndrome (also known as Hughes syndrome). Postivie tests result should be seen on two separate occasions at least twelve weeks apart in order for diagnosis. Antiphospholipid antibody syndrome is an important marker for recurrent thrombosis, and often warrants indefinite anticoagulant (blood thinner) therapy.

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