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ELISA
ACA is short for anticardiolipin antibody, one of three antibody tests doctors use to check for antiphospholipid syndrome, a condition in which the immune system makes proteins that raise the risk of blood clots and pregnancy loss. The lab measures two separate antibody classes, IgG and IgM, using an ELISA plate test rather than a clotting-time test. Doctors typically order it after an unexplained clot in a vein or artery, particularly in someone under fifty, or after repeated miscarriages. A single positive result does not confirm the syndrome on its own - guidelines call for the test to be repeated after twelve weeks, since infections and some medicines can raise the antibody briefly without lasting clot risk.
Your report shows separate IgG and IgM anticardiolipin levels, usually as a number of GPL or MPL units alongside a positive or negative call. A positive IgG result carries more weight for clotting risk than an isolated low positive IgM. The antibody is measured against cardiolipin, a phospholipid found in cell membranes and in mitochondria, which is why it can interfere with normal blood vessel lining and clotting control. Doctors read ACA alongside lupus anticoagulant and anti-beta-2-glycoprotein I results rather than on its own, since antiphospholipid syndrome is defined by at least one of these three tests being persistently positive.
Antiphospholipid syndrome is diagnosed using three separate laboratory tests: lupus anticoagulant, anticardiolipin antibody, and anti-beta-2-glycoprotein I antibody. None of the three is used alone. A person can be positive for one, two, or all three, and having more than one positive test is generally linked to a higher clotting risk than a single positive result.
People with a persistently positive ACA are at higher risk of blood clots in the legs, lungs, or brain, and women with the antibody face a higher risk of miscarriage, especially later in pregnancy. Antiphospholipid syndrome often develops alongside lupus, and doctors managing a lupus patient with a history of clotting or pregnancy loss will often add ACA to the antibody panel.
A single ACA result taken during or soon after an infection can be positive without any connection to antiphospholipid syndrome. That's why current diagnostic criteria require a positive result on two occasions, at least twelve weeks apart, before the antibody is treated as clinically meaningful. A rare but serious complication, catastrophic antiphospholipid syndrome, involves clots forming in multiple organs at once and carries a high mortality rate even with treatment.
Labs report anticardiolipin IgG and IgM separately, usually as a number of GPL or MPL units with a cutoff set by that lab's assay. Ranges vary by lab, so use the reference value printed on your own report rather than a number from another source.
A single positive result can reflect a temporary rise from infection or certain medicines rather than antiphospholipid syndrome. Current diagnostic criteria call for two positive results at least twelve weeks apart before the antibody is treated as clinically significant.
No. ACA is measured with an ELISA plate test that detects the antibody directly, while lupus anticoagulant is measured indirectly through how long your blood takes to clot in the lab. Doctors usually order both, since a person can be positive for one without the other.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
Yes. Infections, some medicines, and even age can cause a temporary positive result without any lasting clotting risk. That's one reason a single positive test is followed up rather than acted on immediately.
There's no fixed fasting requirement tied to this specific test. Ask your doctor or the centre when you book, since instructions can change if other tests are being drawn from the same sample.
Not necessarily. A positive result raises your statistical risk, but many people carry the antibody without ever forming a clot. Your doctor weighs the result alongside your medical history and the other antiphospholipid tests before deciding on next steps.
Antiphospholipid syndrome can occur on its own, without lupus, particularly after an unexplained clot or repeated pregnancy loss. Doctors order ACA in that situation, not only in people already diagnosed with an autoimmune disease.