The Test Sample
What is being tested?
Lupus anticoagulants (LA) are autoantibodies produced by the immune system that mistakenly attack certain components of the body's own cells. They specifically target phospholipids as well as the proteins associated with phospholipids that are found in the outer-most layer of cells (cell membranes). These autoantibodies interfere with the blood clotting process in a way that is not fully understood and increase a person's risk of developing a blood clot. Lupus anticoagulant testing is a series of tests that detect the presence of LA in the blood.
The lupus anticoagulant test's name may seem odd or confusing for two reasons:
- Lupus anticoagulants were so-named because they were first found among patients with lupus, but LA testing is not used to diagnose the autoimmune disorder and LA are frequently absent in people with lupus. LA may also occur in individuals with other conditions and in people who take certain medications. The antibodies are present in about 2-4% of the general population and may develop in people with no known risk factors.
- The term "anticoagulant" is part of the name because LA actually reduce clotting in laboratory tests that are used to evaluate coagulation. For example, they inhibit the chemical reactions that lead to clotting in the partial thromboplastin time (PTT), a test routinely used to evaluate clotting. However, the presence of LA in the human body is associated with an increased risk of developing inappropriate blood clots. Importantly, lupus anticoagulant itself does not cause bleeding in the body.
There is no single test for the detection of lupus anticoagulant and it cannot be measured directly. The presence of LA is usually determined by using a panel of sequential tests for which there is no standardization.
- Initial testing typically involves one or more tests that depend on phospholipid reagents, usually PTT, the LA-sensitive PTT (PTT-LA) or dilute Russell viper venom test (DRVVT). All of these tests measure the time it takes (in seconds) for a sample to clot; LA prolongs that time.
- Depending on the results of these initial tests, certain follow-up tests are performed to either confirm or exclude the presence of lupus anticoagulant. For more on this, see the section titled "How is it used?".
LA may increase the risk of developing blood clots in both the veins and arteries, often in the veins in the legs (deep vein thrombosis, DVT). These clots may block blood flow in any part of the body, leading to stroke, heart attack, or pulmonary embolism. LA is also associated with recurrent miscarriages. It has been suggested that LA causes clots to form that block blood vessels of the placenta, affecting growth of the developing baby, and that LA may also directly attack the tissue of the placenta, affecting its development.
The lupus anticoagulant is one of three primary antiphospholipid antibodies that are associated with an increased risk of thrombosis and antiphospholipid antibody syndrome (APS), an autoimmune disorder characterized by excess blood clot formation and pregnancy complications. The other two are cardiolipin antibodies and beta-2 glycoprotein 1 antibody. Individually and together, they increase a person's tendency to clot inappropriately. People with APS are at greater risk for clotting if they test positive for all three antibodies. However, thrombosis appears more common in people with LA.
Not everyone with antiphospholipid antibodies will develop symptoms. Antiphospholipid antibodies are present in about 5% of healthy individuals.
How is the sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.