A doctor has asked for a lupus anticoagulant test, perhaps after a clot or a pregnancy loss, and the name alone is enough to worry anyone. It has one of the most confusing names in the laboratory, and the reality is simpler than it sounds.
It is not a test for lupus, and a positive result does not mean your blood is too thin. It detects an antibody that interferes with clotting tests in the tube but raises the risk of clots in the body. It is one of the three antibody tests used to diagnose antiphospholipid syndrome, alongside anticardiolipin and anti-beta-2-glycoprotein I antibodies.
Blood London offers the lupus anticoagulant test at 117a Harley Street on a self-request basis, and you can book it online for £98.75 plus the £50 blood draw. Results are usually back within two working days.
Why the test is ordered
- A blood clot in a leg vein, lung or unusual site, especially at a young age or without an obvious cause.
- Recurrent miscarriage, a late pregnancy loss, or severe early pre-eclampsia.
- A stroke or transient ischaemic attack under the age of 50.
- An unexpectedly prolonged APTT clotting time found on routine tests.
- Known lupus or another autoimmune condition, where antiphospholipid antibodies are more common.
How the test works
Lupus anticoagulant is measured indirectly. The laboratory runs clotting tests that depend on phospholipid, such as the dilute Russell viper venom time and a sensitive APTT, then repeats them with added phospholipid. If the clotting time is prolonged and corrects when phospholipid is added, but not when normal plasma is mixed in, the pattern points to a lupus anticoagulant. Because it is a functional clotting test, anything that affects clotting can affect the result.
Anticoagulants and timing
This is the practical point most people miss. Warfarin, heparin and the newer direct oral anticoagulants such as apixaban, rivaroxaban, edoxaban and dabigatran can all cause a false positive or make the test uninterpretable. If you take one of these, the test is usually done before treatment starts, or timed around doses on specialist advice. Keep taking your anticoagulant until you've spoken to the clinician who prescribes it.
Acute clots, infections and pregnancy can also cause a temporary positive. That's why a positive lupus anticoagulant is only confirmed if it is still present on a second sample taken at least twelve weeks later.
What the result means
A negative result on a properly timed sample makes antiphospholipid syndrome unlikely, although the other two antibody tests are still needed to exclude it.
A positive result that persists at twelve weeks, together with a clot or a qualifying pregnancy complication, meets the criteria for antiphospholipid syndrome. It is managed by a haematologist or rheumatologist and usually involves long-term anticoagulation. A single positive result without symptoms is discussed with you rather than treated.
Getting the full picture
If you want the full antiphospholipid antibody set, ask for anticardiolipin and anti-beta-2-glycoprotein I antibodies as well when you book. The thrombophilia screen goes further and includes the inherited clotting factors too.
Frequently asked questions
Do I need to fast?
No. There is no fasting or dietary preparation. Tell the clinic about any anticoagulant or antiplatelet medicine before the sample is taken.
Does a positive result mean I have lupus?
No. The antibody was first described in people with lupus, which is where the name comes from, but most people with a lupus anticoagulant do not have lupus.
Why does it need repeating?
Infections, clots and pregnancy can cause a temporary positive. International criteria require the antibody to persist on a second sample at least twelve weeks apart before antiphospholipid syndrome is diagnosed.