A dilute Russell's viper venom time (dRVVT) is a useful test in the workup of LAC. This venom directly activates factors V and X. The patient's plasma normally gives a standard clotting time of 23–27 seconds. However, if LAC is present, the antibody will interfere with clot activation in the test and prolong the clotting time.
If the dRVVT is prolonged, then normal plasma is added to the patient’s plasma, and the test is performed again. If the prolonged time was due to a factor deficiency, the time will correct. If the clotting time remains prolonged, then this suggests the presence of LAC. A confirmation test is then done in which excess phospholipids are added to the mix. The phospholipids bind the LAC in the patient's plasma, and clotting time should normalize. The time without phospholipid excess is compared to the clotting time with phospholipid excess. If the ratio is >1.3, it is positive for LAC.
LAC is not a coagulopathy. It is an in vitro lab phenomenon. However, LAC's presence is a potential criterion for APS, and APS is a serious coagulopathy.
