Lupus Anticoagulant

Lupus anticoagulant is an immunoglobulin that prolongs phospholipid dependent clotting tests in vitro by interfering with the assembly of coagulation complexes on phospholipid surfaces. It is detected by functional clotting assays and not by immunoassay. Despite the name, it is not a cause of bleeding in most patients. It is associated with arterial and venous thrombosis and pregnancy morbidity, as part of antiphospholipid syndrome. The antibodies are directed mainly against beta-2 glycoprotein I and prothrombin.

Two tests based on different principles are needed, because no single test has sufficient sensitivity: the dilute Russell viper venom time (dRVVT) and a lupus sensitive aPTT (commonly with silica as activator). Each is performed in three steps, namely screening, mixing and confirmation.

Lupus anticoagulant is found in about 1 to 5% of healthy adults at low titre, more often as a transient result after infection. It is found in up to 30 to 40% of people with systemic lupus erythematosus.

Indications. Unexplained venous or arterial thrombosis (particularly in people under 50 years, at unusual sites or recurrent), stroke or transient ischaemic attack in young adults, recurrent early pregnancy loss (3 or more before 10 weeks), foetal death at or after 10 weeks, severe pre-eclampsia or placental insufficiency leading to delivery before 34 weeks, thrombocytopenia with a prolonged aPTT, livedo reticularis, a systemic lupus erythematosus workup, an unexplained prolonged aPTT, and before surgery in a patient with a prolonged aPTT.

Specimen.

• Citrated blood, centrifuged twice to give platelet poor plasma with a platelet count under 10 x10⁹/L, because platelet phospholipid neutralises the antibody.
• Plasma frozen at minus 70 °C if the test is delayed over 4 hours, and thawed rapidly at 37 °C. Aliquots are stored in plastic tubes.
• Timing should account for anticoagulant therapy.

Step 1: screening. Prolonged dRVVT or lupus sensitive aPTT, using low phospholipid concentrations. The screen is read as a ratio to a normal pooled plasma and compared with a locally established cut off at the 99th percentile of healthy controls. A typical dRVVT ratio over about 1.2 is abnormal, and the cut off differs by reagent.

Step 2: mixing. Equal parts of patient and normal pooled plasma. Failure to correct (a ratio, or index of circulating anticoagulant, over the local cut off) shows an inhibitor and excludes a factor deficiency. Correction suggests deficiency, though a weak lupus anticoagulant can correct. An incubated mix is used for time dependent inhibitors.

Step 3: confirmation. The test is repeated with a high concentration of phospholipid. A shortening of the clotting time (a normalised screen to confirm ratio over about 1.2 for dRVVT, with a cut off set locally) shows phospholipid dependence, which confirms lupus anticoagulant.

Result. Positive when at least one assay shows a prolonged screen, an inhibitor on mixing and phospholipid dependence on confirmation. A negative result on both assays excludes lupus anticoagulant if the sample is adequate.

Persistence. A positive result is repeated at least 12 weeks later. Persistence defines the laboratory criterion for antiphospholipid syndrome, and testing is paired with anticardiolipin and anti-beta-2 glycoprotein I antibodies.

Clinical significance.

• Lupus anticoagulant carries the strongest association with thrombosis and pregnancy loss of the antiphospholipid tests.
• Triple positivity (lupus anticoagulant, anticardiolipin and anti-beta-2 glycoprotein I) carries the highest risk of thrombosis, and recurrence is greater after the first event.
• A high titre antibody, or a persistent positive in a patient with a thrombotic or obstetric event, supports antiphospholipid syndrome.
• Transient positivity after infection, with a risk of confusion, is common and often clinically silent.

Causes of a positive result. Primary antiphospholipid syndrome, systemic lupus erythematosus and other autoimmune diseases, infection (HIV, hepatitis C, syphilis, malaria, tuberculosis, viral infection in children), malignancy (lymphoproliferative disease), drugs (chlorpromazine, procainamide, hydralazine, phenytoin, quinine, some antibiotics), and healthy carriers.

Bleeding with lupus anticoagulant. Rarely seen, and mainly with associated hypoprothrombinaemia (acquired factor II deficiency from antiprothrombin antibodies), thrombocytopenia, or acquired inhibitors.

Special reporting. The strength of the effect is reported as positive, weak or strong, and the assays used are noted.

Critical findings needing immediate action.

• A positive result in a patient with acute multi-organ thrombosis over days to weeks needs immediate assessment for catastrophic antiphospholipid syndrome, a medical emergency.
• A positive result in pregnancy needs early obstetric and haematology planning for prophylaxis.
• A prolonged aPTT with unexplained bleeding needs an inhibitor screen to exclude acquired haemophilia, and a lupus anticoagulant must not be assumed.

Interferences and pitfalls.

• Unfractionated heparin up to about 1 IU/mL is neutralised by reagents, and higher concentrations cause false positives. Low molecular weight heparin has little effect if sampled at trough.
• Warfarin prolongs the screen. Test when the INR is under about 1.5 where possible, or use a 1 to 1 mix, and do not stop anticoagulant for the test alone in a patient at high thrombotic risk.
• Direct oral anticoagulants (rivaroxaban, apixaban, edoxaban, dabigatran) cause false positive dRVVT results, and the confirmatory test can fail. Testing at trough, and a DOAC removal agent, reduce the problem, and testing is often best delayed until anticoagulant is stopped where safe.
• C-reactive protein over about 50 mg/L can cause false positives in some aPTT reagents.
• Acute thrombosis, pregnancy, and an acute phase response can alter the result, and a positive test during acute illness is repeated.
• Factor deficiencies, specific factor inhibitors and fibrinogen disorders prolong the screen, and mixing and confirmatory steps separate them.
• Under centrifuged plasma with residual platelets gives a falsely negative result.
• Reagent sensitivity varies, and two methods increase sensitivity.

Special groups.

• Pregnancy: lupus anticoagulant increases risk of loss and placental disease, and results need testing in specialist services. Testing can be done during pregnancy but interpret with care, since factor VIII and fibrinogen rise.
• Children: transient lupus anticoagulants after infection are common, produce a prolonged aPTT without bleeding, and often resolve. Persistence is assessed at 12 weeks.
• Older adults: positive results are more frequent without clinical syndrome.
• Patients on anticoagulants: testing is complex and directed by a specialist.

Follow up tests. Anticardiolipin and anti-beta-2 glycoprotein I antibodies, repeat lupus anticoagulant after 12 weeks, antinuclear antibodies, anti-dsDNA, complement, full blood count and platelets, clotting screen, factor VIII, IX and XI assays, prothrombin activity, syphilis and HIV serology, direct antiglobulin test, renal function with urine protein, imaging for thrombosis.