Lupus Anticoagulant Screen
Haematology Overview / Directory
Purpose of the test
Lupus anticoagulants (LA) are also known as anti-phospholipid antibodies. Their presence is known to cause arterial and venous thrombosis, recurrent spontaneous abortions, thrombocytopaenia and neurological problems. They are found in association with auto-immune conditions, exposure to some drugs, some infections, and lymphoproliferative disorders. The antibodies are a heterogeneous group which interfere with the action of anionic phospholipids. This interference can be demonstrated by prolongation of clotting times in phospholipid-dependent clotting tests, such as the Activated Partial Thromboplastin Time (APTT) and the dilute Russell’s Viper Venom test (DRVV). In contrast, the antibodies cause thrombosis in vivo. The DRVV test has been shown to be more specific for lupus anticoagulants because it is not influenced by intrinsic factor deficiencies or intrinsic factor inhibitors which cause prolongation of the APTT.
Russell’s Viper Venom activates Factor X directly, thus bypassing the intrinsic and extrinsic coagulation pathways. Coagulation is via the common pathway and prothrombinase complex formation which is dependent on phospholipid for thrombin generation.
In the DRVV test the reagent contains dilute RVV, plant phospholipid and calcium ions. The reagent is added to the patient’s plasma and resulting clot formation is timed. Antiphospholipid antibodies, if present, will neutralise the phospholipid component of the reagent thus inhibiting prothrombinase complex formation, resulting in a prolongation of the clotting time.
The result is expressed as a ratio of the patient’s DRVV clotting time over the DRVV clotting time of normal plasma.
If the DRVV ratio is raised, the presence of a Lupus Anticoagulant is confirmed by repeating the test using a reagent containing DRVV, calcium ions and a high concentration phospholipid which will neutralise any LA present so that thrombin formation is not inhibited. The confirmatory test is performed on patient and normal plasmas, the ratio is calculated and the result reported as the percentage correction of DRVV Test ratio
Reference Range
DVVTest ratio <1.15 is negative for Lupus Anticoagulant.
DVVTest ratio >1.20 AND
% correction of ratio >12% = positive for Lupus Anticoagulant
Sample Requirements
2 tri-sodium citrate samples.
Storage and Transport
Samples must kept at room temperature and sent directly to the Haemostasis laboratory to be processed within 2 hours of receipt.
Turnaround Time
The assay is run every week.
Factors affecting results or interpretation
Clots of any size , haemolysis, underfilling or overfilling will affect result.
Certain anticoagulants such as, warfarin, will affect interpretation of results.
Contacts
Haemostasis Laboratory on 020 3299 9000 ext 2434
e-mail: liz.ford@nhs.net
