High levels of antiphospholipid (aPL) antibodies in the blood are associated with increased risk of thrombosis and serious problems during pregnancy. One way to measure aPL antibodies is by functional tests called lupus anticoagulation. However Lupus anticoagulant interpretation can be difficult.
What does Lupus mean?
The term Lupus comes from the autoimmune disease Systemic Lupus Erythematosus (SLE). It is so named because patients have a characteristic rash across their face which is said to make them look ‘wolf-like’ or Lupine. Patients with SLE often have high aPL antibodies.
What is Anticoagulation?
Coagulation is the normal biochemical process of clotting that occurs in response to injury. It is essential to minimize blood loss, and to allow healing and repair of damaged tissues. Clotting can occur by a variety of complex mechanisms which require numerous clotting/coagulation factors and ends in the formation of a fibrin clot.
Some clotting pathways are 'phospholipid dependant', therefore will only occur if phospholipids (PL) are present. The presence of high levels of antiphospholipid antibodies (aPL) hinders this coagulation process, in other words aPL have an anticoagulation effect. It is this anticoagulation, or slowing of coagulation, which is measured by the lupus anticoagulation (LA) test.
Tests for Lupus Anticoagulants
The most commonly performed LA tests are:
- Dilute Russel Viper Venom Time (dRVVT)
- Activated Partial Thromboplastin Time (APTT)
- Prothrombin Time Test (PT)
At least two of these test should be performed. In addition to measurements of LA activity, the clinician may also request direct measurement of aPLs.
Lupus Anticoagulant Interpretation
Recommendations on lupus anticoagulant interpretation by the International Society on Thrombosis and Hemostasis state that a lab report of prolonged LA activity should only be given if:
- One or more of the PL dependant coagulation times must be prolonged
- It must be established that the prolonged time is due to the presence of an inhibitor of clotting, not due to the lack of a clotting factor.
- The inhibitor must be inhibiting the function of the PLs, not inhibiting a clotting factor.
- LA tests should be carried on two occasions, at least 12 weeks apart, as temporary raised aPL antibodies can result from some infectious diseases.
Reasons for Lupus Anticoagulation Testing
- Patients suspected of autoimmune conditions such as SLE and rheumatoid arthritis.
- Women thought at risk of miscarriage or other pregnancy complications.
- Patients with thrombotic disease: the LA test measures the slowing of the coagulation process, but paradoxically, aPLs can have the opposite effect in the body. By binding blood vessel walls, aPLs can encourage pathological clot formation, causing thrombotic disease.
- LA may be used to monitor the effects of drug treatment.
Limitations of LA Testing
- False positive results can occur, hence it is important that a diagnosis is not made from a single test.
- Patients being given anti-coagulation therapy such as vitamin K antagonists may show prolonged coagulation times due to their treatment. This can make the detection of LA activity in serum difficult.
- Exact quantification LAs are not reliable with currently available laboratory methods.