Lupus Anticoagulant

A clotting-time test for lupus anticoagulant, an antibody effect that slows clotting in the lab even though it is linked to clots in the body.

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What it is

Lupus anticoagulant (LA) is not a blood thinner, and it does not by itself mean you have lupus. It is the name for antibodies that interfere with clotting tests that depend on phospholipids, which are fatty molecules. Labs look for it with at least two clot-based tests, usually a dilute Russell's viper venom time (dRVVT) and a silica-based aPTT that is sensitive to LA. If a first test is long, the lab repeats it after mixing in normal plasma and again with extra phospholipid. That helps show whether LA, a blood thinner or a missing clotting factor is the reason. The result is usually written as an interpretation, such as detected, not detected or indeterminate.

Why it matters

LA is one of three lab findings used to identify antiphospholipid syndrome (APS), a condition linked to blood clots and to some pregnancy problems. The other two findings are anticardiolipin and anti-beta-2 glycoprotein I antibodies. One positive LA result is not enough on its own. It has to be found again when repeated at least 12 weeks later, and it is read together with your symptoms and history. Doctors often order it after a clot with no clear cause, after repeated pregnancy loss, or when a routine aPTT is longer than expected.

If it’s high

Detected means the lab saw the clotting pattern of a lupus anticoagulant. This is not a diagnosis. The result is repeated at least 12 weeks later to see whether it stays, and it is read with your history and other antibody tests. Blood thinners, especially direct oral anticoagulants, can create a false pattern, so tell the lab what you take.

Common causes

A blood thinner in the sample (heparin, warfarin or a direct oral anticoagulant) that makes the test look positive
Testing during an acute illness, a recent clot or pregnancy, when other proteins in the blood can interfere
Antiphospholipid syndrome, on its own or alongside lupus or another autoimmune condition

What can help

Ask your clinician when to repeat the test. At least 12 weeks later is the usual way to check that it stays.
Ask whether anticardiolipin and anti-beta-2 glycoprotein I antibody tests have been done
Tell your clinician and the lab about every blood thinner you take. Do not stop or change any on your own.

What can affect your result

Blood thinners: heparin, warfarin and direct oral anticoagulants can cause false-positive or false-negative patterns
Timing: an acute illness, a recent clot or pregnancy
Sample quality: platelets left in the plasma, an underfilled tube or a clotted sample
The reagents and cutoffs your lab uses, so two labs can disagree on a weak result
A clotting factor shortage or a factor inhibitor, which can also lengthen the first screening test
Blood thinners can change the result in either direction. Heparin (including low-molecular-weight heparin), warfarin and direct oral anticoagulants such as rivaroxaban, apixaban and dabigatran can cause false-positive or false-negative patterns. Some reagents neutralize heparin, but only up to about 1 U/mL. Tell the lab which blood thinner you take, and never stop one on your own to get tested.
Timing matters. Testing is best avoided during an acute clot or during pregnancy, because acute-phase and clotting proteins in the blood can interfere. If it has to be done then, it is usually repeated afterward.
The sample must be platelet-poor plasma that was spun twice. Platelets left in the sample, or freezing and thawing done poorly, can release phospholipids that hide a weak lupus anticoagulant.
An underfilled or clotted collection tube can spoil the result.
A shortage of a clotting factor or a factor inhibitor can also lengthen the first screening test. The mixing and confirmation steps are meant to tell these apart from a lupus anticoagulant.
Labs use different reagents and cutoffs, so two labs can disagree on a weak result near the cutoff.

This is not a level that changes with lifestyle. A lupus anticoagulant can be found once and not later, so persistence is checked by repeating the test after at least 12 weeks.

When to talk to a clinician

A detected result is not an emergency, but ask the clinician who ordered it to review it with you. Call 911 or your local emergency number for possible signs of a clot: sudden chest pain or shortness of breath, a swollen, painful or warm leg, sudden weakness or numbness on one side, trouble speaking, or a sudden severe headache.

Questions you could ask

Was this a good time to test, given my recent illness, clot or pregnancy?
Does my lab account for the blood thinner I take?
Should we repeat this test in 12 weeks or more?
Have anticardiolipin and beta-2 glycoprotein I antibody tests been done?
Could a medicine I take, including a blood thinner, have affected this result?

Questions people ask

Does a positive lupus anticoagulant mean I have lupus?

Not by itself. The name is historical. LA is one lab sign used to identify antiphospholipid syndrome, which can happen with or without lupus. Your clinician looks at your symptoms and other tests.

Why would my clinician repeat the test after 12 weeks?

A lupus anticoagulant can be present once and gone later. To count toward a diagnosis of antiphospholipid syndrome, it has to be found again after at least 12 weeks.

Can my blood thinner change the result?

Yes. Heparin, warfarin and direct oral anticoagulants can all affect LA tests and may cause false positives or false negatives. Labs can sometimes neutralize part of the effect, so tell the lab and your clinician what you take, and let them decide on timing. Never stop a blood thinner on your own.

Show the science and sources

For clinicians and researchers

LA is detected by clot-based, uncalibrated assays, not measured as an amount. The ISTH Scientific and Standardization Committee guidance (Devreese 2020), as summarized in a 2024 review, uses a three-step process on platelet-poor citrated plasma: screening with two tests based on different principles, a dRVVT and a lupus-sensitive aPTT (silica is the preferred activator); a mixing step; and a confirmatory step with excess phospholipid. Each lab sets its own cutoffs, and a screen-to-confirm ratio of about 1.2 is the default cutoff used by most participants in an Australasian external quality program. Mayo's profile adds mixing and confirmation when the dRVVT ratio is 1.20 or higher or the aPTT is 38 seconds or more. ARUP reports a PTT-LA ratio, a dRVVT screen ratio and a confirmation ratio, each with a reference of 1.20 or less, and adds anti-Xa, thrombin time and neutralization steps when a blood thinner is suspected. Anticoagulants interfere: some commercial reagents include heparin neutralizers (up to about 1 U/mL), vitamin K antagonists can lengthen the screens, and direct oral anticoagulants can produce false-positive patterns (rivaroxaban, dabigatran) or false negatives (apixaban) unless they are removed with an adsorbent such as DOAC-Stop. Testing is best avoided during acute thrombosis or pregnancy, and a positive result needs repeat testing after at least 12 weeks to show persistence. LA, anticardiolipin and anti-beta-2 glycoprotein I antibodies are read together, with triple-positive patients carrying the highest risk.

LOINC 75882-1 (primary), 3281-3, 75514-0, 75881-3. Specimen: plasma.

Sources

Lupus Anticoagulant Profile, Plasma (test ID ALUPP), Mayo Clinic Laboratories (Extended Catalog, via testcatalog.org)
Lupus Anticoagulant Reflex Panel (test 3017009), ARUP Laboratories
Testing for the lupus anticoagulant: the good, the bad, and the ugly, Research and Practice in Thrombosis and Haemostasis (2024)
Guidance from the Scientific and Standardization Committee for lupus anticoagulant/antiphospholipid antibodies of the International Society on Thrombosis and Haemostasis: Update of the guidelines for lupus anticoagulant detection and interpretation, Journal of Thrombosis and Haemostasis (2020)
Pre-Analytical Variables, Practical-Haemostasis.com

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.