The LA-sensitive PTT (LA-PTT or PTT-LA) is a variant PTT, designed to evaluate the presence of lupus anticoagulant (LA), an antibody associated with clotting episodes and recurrent miscarriages. The LA-PTT uses a low phospholipid reagent that is optimized for detecting lupus anticoagulants and is therefore more sensitive to LA. The test is based on the principle that lupus anticoagulant binds to the phospholipids that are used as one of the reagents in the PTT test, causing an abnormally prolonged clotting time.

Lupus anticoagulants (LA) are antibodies 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 term “anticoagulant” is part of the name because LA actually prolong clotting time 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-containing reagents, usually PTT, the LA-sensitive PTT (known as PTT-LA) or dilute Russell viper venom test (DRVVT). All of these tests measure the time it takes (in seconds) for a plasma 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.

LA may increase the risk of developing blood clots in both the veins and arteries, often in the veins in the legs (known as deep vein thrombosis or 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, organ failures, 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.