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.^[1] The inhibitory effect of LA on dRVVT can be overcome by adding an excess of phospholipid to the assay – this can be used as a specificity control to confirm the diagnosis, as are mixing studies, which confirm that clot prolongation is due to an inhibitor, rather than a factor deficiency. 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.^[2]