LA Ratio (Dilute Russel's Viper Venom Time; DRVVT)

Lupus anticoagulants (LA) occur in various clinical conditions, especially autoimmune diseases. LA are a significant risk factor in patients with otherwise unexplained thrombosis and are often present in women who have recurrent miscarriages.

They are autoantibodies against negatively charged phospholipids bound to proteins and have traditionally been detected using phospholipid-dependent clotting tests such as the activated partial thromboplastin time (APTT) or dilute Russell's Viper Venom time (DRVVT). A prolonged clotting time which is not corrected by mixing patient plasma with normal plasma usually indicates a circulating anticoagulant. However, correction of a prolonged clotting time with phospholipid is a more specific characteristic of LA.

Russell's Viper Venom directly activates factor X and may be more specific for LA than tests which are also sensitive to contact factors. In the LA Ratio test, a DRVVT time is run with (CONFIRM test) and without (SCREEN test) excess phospholipid. In the presence of a LA, the clot time of the SCREEN test may be significantly prolonged. However, the clot time will be largely normalized in the CONFIRM test due to neutralization of the LA by the phospholipid. The RATIO of the two clot times (normally ~1.0) will therefore be increased.

In the Hemostasis and Thrombosis Laboratory, the normal range is 0.80 - 1.44, The presence of a LA is suggested by a LA Ratio >=1.45.

It is important to note that not all LA will be detected by the LA Ratio (DRVVT). Following recommendations of the International Society on Thrombosis and Haemostasis, three independent test systems should be included when testing for Lupus Anticoagulants. Therefore, whenever the LA Ratio (DRVVT) test is ordered, the Hemostasis and Thrombosis Laboratory will also perform a Kaolin Clot Time and PTT-LA to comply with the suggested three-test system.