What is being tested?
Antiphospholipid antibody tests are used to detect specific autoantibodies, proteins the body creates against itself in an autoimmune response to phospholipids. Found in cell membranes and platelets, phospholipids are a normal part of the body. They are lipid molecules that play a crucial role in blood clotting. When antiphospholipid antibodies are produced, they interfere with the clotting process in a way that is not fully understood. They increase an affected patient’s risk of developing recurrent inappropriate blood clots (thrombi) in arteries and veins, which can lead to strokes and heart attacks. Antiphospholipid antibodies are also associated with thrombocytopenia (decreased platelets) and with the risk of recurrent miscarriages (especially in the 2nd and 3rd trimester), premature labour, and pre-eclampsia.
One or more antiphospholipid antibodies are frequently seen with autoimmune disorders such as systemic lupus erythematosus (SLE). They may also be seen with HIV, some cancers, temporarily with infections and some drug treatments (such as phenothiazines and procainamide), and in the elderly. Antiphospholipid syndrome (APS), also called Hughes syndrome, is a recognised group of signs and symptoms that includes the formation of thrombi, miscarriages, thrombocytopenia and the presence of one or more antiphospholipid antibodies. APS can be primary (with no underlying autoimmune disorder) or secondary (existing with a diagnosed autoimmune disorder).
The most common antiphospholipid antibodies are cardiolipin antibodies (also called anticardiolipin antibodies), lupus anticoagulant and anti-beta-2 glycoprotein 1. Others that are not as frequently tested include anti-phosphatidylserine. While the lupus anticoagulant cannot be tested directly, several classes (IgG, IgM, and/or IgA) of the other antibodies can be measured. Each of these tests contribute information that either leads toward the confirmation of the presence of clinically significant antiphospholipid antibodies or not. While recent recommendations have emphasised the use of both a lupus anticoagulant test and the anti-beta-2 glycoprotein 1 test for the classification of antiphospholipid syndromes, anticardiolipin antibodies are also included in the diagnostic work-up. It is likely that IgM antibodies are not pathogenic but they remain in the international consensus criteria until further data is accumulated.
The lupus anticoagulant is detected by performing a panel of clotting based tests that begin with a prolonged APTT (which indicates the presence of a substance interfering with the clotting process), followed by a mixing study in which patient plasma and normal plasma are mixed, to confirm that the prolongation of APTT is due to inhibitor and not to a deficiency of coagulation factors. Confirmatory studies need to be performed, preferably with a similar method as the initial screening assay. These confirmatory tests may include RVTT (Russell viper venom time), PNP (platelet neutralisation procedure), KCT (kaolin clotting time) and/or hexagonal lipid neutralisation test.
How is it used?
Antiphospholipid testing is used to help determine the cause of an unexplained thrombotic episode, recurrent fetal loss, thrombocytopenia, and/or a prolonged APTT test. Depending on the patient’s clinical findings, a physician may order one or more types and classes of these tests to help detect the presence of antiphospholipid antibodies and to help diagnose antiphospholipid syndrome (APS). Cardiolipin antibodies (IgG, IgM, and sometimes IgA) are frequently ordered as they are most common antiphospholipids. If a patient has a prolonged APTT test, further lupus anticoagulant testing is usually indicated. The lupus anticoagulant may be present when the APTT test is normal. Anti-beta-2 glycoprotein 1 should be ordered along with the other antiphospholipid antibodies to detect their presence and to provide the doctor with additional information.
If an antiphospholipid antibody is detected, the same test(s) may be ordered 10 to 12 weeks later to determine whether their presence is persistent or temporary. If a patient with an autoimmune disorder tests negative for antiphospholipid antibodies, they may be retested as these antibodies may develop later. It is not unusual for antiphospholipid antibody levels (titres) to fluctuate significantly in a single person over time.
When is it requested?
Antiphospholipid antibody testing may be ordered when a patient has a diagnosed thrombotic event. It also may be ordered when a woman has had recurrent miscarriages and/or as a follow-up to a prolonged APTT test. When one of the tests is positive, it may be repeated several weeks later to determine whether the antibody is temporary or persistent. Antiphospholipid testing may be done when clinical signs suggest the presence of antiphospholipid syndrome. When a patient with an autoimmune disorder tests negative for antiphospholipid antibodies, one or more of the tests may be ordered in the future to screen for the development of an antiphospholipid antibody.
What does the result mean?
Care must be taken when interpreting the results of antiphospholipid antibody tests. A negative result means only that the specific antibody tested was not present at the time of the test. Low to moderate levels of one or more antibodies may occur temporarily due to an infection or drug or may appear as a person ages. These concentrations are often not considered significant but must be examined in conjunction with a patient’s symptoms and other clinical information. In some cases, a person may have one or more classes of a specific antibody present or absent. For instance, they may have significant quantities of IgG and IgM cardiolipin antibodies or they may only be positive for the less frequently tested IgA cardiolipin antibody. Moderate to high levels of one or more antiphospholipid antibodies, which persist when tested again 8 to 10 weeks later, indicate the likely continued presence of that specific antibody. Isolated IgM antiphospholipid antibodies are not likely to have clinical significance.
If the tests indicate the presence of the lupus anticoagulant and it persists when retested, then it is likely that the patient is positive for the lupus anticoagulant. Patients who have one or more antiphospholipid antibodies and those that are diagnosed with antiphospholipid syndrome have an increased risk of recurrent thrombotic episodes, recurrent miscarriages and thrombocytopenia. Test results cannot predict, however, the likelihood of complications, the type or the severity in a particular patient. Some will have a variety of recurrent problems while others may never experience any difficulties. Examples of this include an asymptomatic patient who is diagnosed with antiphospholipid antibodies following a prolonged APTT test that is done for another reason (such as a pre-surgical screen), and an asymptomatic elderly person who has acquired an antiphospholipid antibody.
Is there anything else I should know?
Occasionally, antiphospholipid testing may be ordered to help determine the cause of a positive VDRL/RPR test for syphilis. The reagents (chemicals) used to test for syphilis contain phospholipids and can cause a false positive result in patients with antiphospholipid antibodies.
Common questions
No. The process that causes them to be produced is not well understood. There are controllable risk factors, however, such as smoking and obesity that also increase your risk of clotting. Addressing these issues may help to lower your overall risk of developing thrombi.
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